There wasn't exactly a lot of engagement for this one, but I made up some of my own and figured I'd assemble more in the meantime. Some of them are just random ones, but I did add some that were common to the DP fandom too. Hope you enjoy!

Triple AU:

1. When the accident started, the only difference was that the blast doors weren't closed and the energy hit all three of them after they tried getting Karkat out of the way of the blast. All three of them half-die in the blast, resulting in a trio of kids getting superpowers, Spades finding them, and panic ensuing for everyone.

2. The situation is a nightmare for everyone, but in some ways, it's a lot more ideal compared to the canon universe since they all have powers to mess around with. They all blame themselves for the accident, especially Spades since it was his invention, but at least they aren't alone and are allowed to practice their powers with each other. Not to mention that they can all take equal parts of the fighting rather than Karkat taking the brunt of it while the other two play support.

3. As for how the story changes, it'd be focused heavily on how these three adapt and what kind of enemies they create on their own. A lot of the interactions change too, and each one gets their own nemesis as far as they're concerned, and things would be a lot lighter despite the fact it starts with three teenagers dying. Plus, if Karkat played his cards right, his deal with Dualscar would be moot since technically only he has to not attack him, not his friends.

4. As for what powers each one would have; Aradia would probably gain an earth-based core, which sounds less ideal, but can be used to borderline make earthquakes and change the terrain. It's used as a last resort since she doesn't want to destroy the town. She'd probably also be able to control dead things, mostly bones, as a way to get the numbers advantage. She'd probably earn the title as the scariest one of the trio without even trying, even though she'd occasionally use her powers to play with local kids.

5. Sollux, on the other hand, would gain something closer to electricity (an easy choice due to Homestuck canon, but it works). He'd stick to shooting it and using thunderstorms to his advantage, but if anyone tries to grab him and doesn't have any immunity to it, he'd end up being a living taser. As for his own special power, it's probably related to tech and software. Unlike the Zahhak's, it'd most likely be less useful offensively and sticks mostly to modern technology, but he can use practically any device that doesn't have any ecto-protections on it. He'd definitely have a search engine in his brain, with the only issue being no option for a safe search.

Mom Friend AU:

1. In another world, Rosa was able to keep in contact with Spades and when she was ready to move to Amity Park with her kids, she immediately did so she and her family could at least grow up with Kankri's last living reminder. Admittedly, it isn't a healthy idea, but she makes it work for herself and for the others too.

2. Kanaya and Karkat grow up pretty close due to this alongside Aradia and Sollux. You can almost consider them the best of friends and Karkat trusts her with most things, with her dynamic being the "older sister" or "mom friend" of the group. This is despite the fact that she's not that much older than any of them and is debatably the same age. She gets them out of trouble a lot if she can afford it. The only issue is Karkat feeling a bit of a complex dealing with Rosa and how she sees Kankri in him instead, but that isn't much of a problem until later.

3. As for the accident and how she knows about it, it can happen one of two ways; she knows from the beginning, or she figures out later. For the first, she'd see Karkat die in a horrible accident and somehow get out of it with superpowers, with a whole set of traumas following with it. She'd hold out the longest about the situation, thinking that Karkat's powers will eventually wear off and things will go back to normal again, and eventually breaking down the longer this goes on. She's vehemently against the idea of "Phantom," and desperately tries to get him to stop for his own safety.

4. As for the second scenario, Kanaya only gets suspicious when Karkat seems to be distancing himself from her since he's afraid of how she'll react. When it becomes nearly impossible for her to dig anything up, she gets frustrated and turns her attention to the weird stuff happening in her town. She starts noticing Phantom appearing everywhere and figures she should study him, with the eventual realization that it's Karkat. She practically goes ballistic when she realizes it, knowing the fact that her best friend died and she didn't even notice causing a mental breakdown, and the fact he's putting himself into serious danger pushing her the rest of the way.

5. Honestly, it would make the story a lot more dramatic, especially the longer the Phantom charade goes on. I'd feel like she'd snap eventually, either joining in on the fighting with her own chainsaw or forcing Spades to realize the truth to see if he can stop it. Maybe she'd calm down meeting other ghosts that are on Karkat's side and calm down a little, eventually accepting Karkat has it all under control. Well, at least until she meets Signless…

Alternating Halfas AU:

1. The way these kids into accidents involving interdimensional portals is honestly disturbing, especially in certain respects. Aradia would most likely get in the accident out of curiosity involving the portal but would end up causing a bad chain of events leading to her half-death. Sollux would probably be lured into the portal out of spite, especially if he starts complaining about all the "wasted" tech that was put into making it. Of course, he regrets those words once he finds out how to turn it on successfully.

2. Each one has a very different way of dealing with the fact that they died, and both are flavors of denial. Aradia doesn't really think about it, focusing more on the new powers and all the new things to discover. It works for a while, but it become unavoidable when it starts becoming obvious the powers aren't going away anytime soon. Sollux, though, tries to pretend it never happened and that the powers aren't even real. He basically pulls off what happens right after for Karkat except for much longer, denying the fact ghosts even exist until the cases of the Serkets and Tavros come up.

3. I've talked about it before in another comment, but they would get earth and electricity-based powers respectively. Aradia would be able to control dead things, while Sollux has tech. This is almost against their own respective halfa rivals, with Aradia getting Condy while Sollux gets a halfa version of Darkleer (my first choice, but it could be the Clopin for another substitute). Condy would probably be going after the portal itself because it would provide her power, and Darkleer would try to make more halfa's out of some sense of superiority and wanting to make more for himself to be over.

4. As for how the story goes, it would change somewhat since Aradia and Sollux are different to Karkat in many respects. Aradia would probably poke a few more holes into things because of her natural curiosity and maybe end up meeting more ghosts, good and bad. Sollux would definitely have rougher relations with most ghosts since he never wanted to deal with this in the first place (aversion to change and just generally in denial playing a large part in it). They aren't Blood Aspects, but they still make some allies, as tenuous as they may be.

5. Karkat himself is wracked with guilt in either scenario, figuring he should've known better than let either of them mess around in the portal. He might eventually become an antagonist for a short time depending on how much he blames his father for making the damn thing and almost ruining the relationships with his only two friends and killing one. At the very least, he becomes a bit of a known threat despite being one of the human members anyway.

Divide AU:

1. This reality is much different in comparison to the main universe, with a lot more a crossover vibe to it. Karkat Vantas is the adopted kid of Jack and Maddie Fenton, having adopted him after the death of their friend Kankri, with sibling Jazz and Danny Fenton in the same family. Growing up among lunatics in the town of Amity Park, life was relatively normal for him, despite being something of an outcast and loner among the populace. Of course, this all changes when he and Jazz mess around with their parents portal and Karkat becomes a halfa.

2. The human world deals with Danny Phantom characters, with the exception of Karkat, but the Ghost Zone has all of the Homestuck characters inside, making a clear divide between worlds. It's a mixed bag with plenty of them becoming enemies, but also many different ghosts joining Karkat's side. Many of his friends in canon meet up with him and Karkat is suddenly surrounded by friends, a major change from his loner background. It's all chaotic, but Karkat is able to handle it with Jazz's help, and later the other ghosts.

3. It's a huge clash between two very different worlds. The Ghost Zone is a very different place to this Earth, and is thus seen as much more horrifying when ghosts start attacking it. Meanwhile, Amity Park seems like regular suburbia most of the time and Karkat usually has to survive the perils of it just as much of his normal fights. There's a massive rift between his two lives, and surprisingly, it's a hard time for him to figure out which one he actively prefers.

4. You see, while Karkat loves his parents, his siblings, and even some of Amity Park, the Ghost Zone and the people he meets there feel a lot more like home to him each day. The people of Amity Park feel almost one dimensional, Vlad (who is the halfa adult in this AU) is pathetic and lets his selfishness override any good part of him, and the eventual bad reputation he gets among the town and his own parents drive him mad. Meanwhile, he likes hanging out with ghosts like Aradia and Sollux, meeting up with Kanaya and her family, and just generally finds the world of the Ghost Zone more interesting. While he feels guilty for these thoughts, they only grow stronger day by day.

5. It's generally a mixed bag all around for everyone, especially for those on the human side of things. Jazz is seeing this all happen in real time and realizing how much happier her brother is in a place that isn't home. Danny, who only figures out later about the accident, is outright terrified to figure out what Karkat is becoming, and why his brother is okay with it. Other like Valerie, who Karkat was a lot more straightforward to about the whole thing at Axion, is horrified to see a human "turn into" a ghost in real time. Karkat hasn't told his parents about him and most likely never will.

Justice League AU:

1. Karkat lives in a world of superheroes and supervillains, and he does not want anything to do with it. While he could be pressured into saying his favorite superhero is Aquaman (something he shares with Feferi), he's seen how it can take a toll on any city that happens to be rife with them. Thankfully, despite the fact his dad works in paranormal science, he lives in a small town that isn't likely to attract any of them anytime soon. The accident that turns him into a halfa then decides to wreck that moment of hubris.

2. Karkat is panicking, alright? He doesn't want to be a superhero whatsoever, but he also realizes he's the best person for the job. However, he also knows if he gets famous like certain ones, that'll just attract more villains that want to see what all of the fuss is about. He can't handle that on his own, and his protection obsession is already making him a mess, so he decides early on that, as far as anyone outside of Amity Park knows, there is no ghost problem in his town.

3. Using the power of Sollux's tech skills, Aradia learning magic, and connections he makes later, he is able to keep the secret of ghosts stuck within Amity Park. Stuff like convincing his dad to go over everything with his government contractors are something he manipulates him into doing, any videos that aren't instantly corrupted by ecto-signatures are deleted or not available outside of town, and a spell is placed so people will always forget to mention ghosts when leaving Amity Park. It's a cover job that requires constant surveillance, but for a long while, it works in keeping the problem in one place.

4. This does break slowly over time, though, as even the best cover jobs can't last forever. The occasional magic or occult based hero will pass through and figure that something is not right there, but are eventually persuaded to keep quiet (Constantine and Zatara are likely suspects). There are a few close calls when the more confident/stupid ghosts try to cause a disturbance in other towns, which are shut down by Karkat and company fairly quickly. Eventually, at least one supervillain will find it and blow the lid open, forcing Karkat to show an open and terrifying display of power to cap it off.

5. Whether he finds a Justice League member or one finds him, Karkat does his best to make sure the fallout doesn't become intense, but that there is a clear divide between him and them. If heroes like Superman start getting possessed, it would be a disaster for everyone, and he make sure that point gets across clearly and concisely. Thus starts a very awkward, tense relationship between Amity Park with the rest of the Justice League world, for better or for worse.

+6. Just as an extra; Karkat may or may not resemble a cat-themed thief and Batman is not sure how to deal with that since Selina admitted she may have had a kid with a priest a very long time ago. It doesn't help that she might have a heart attack when she sees him in human form too…

No One Knows AU:

1. Karkat grew up alone. That was a simple fact about his life and he figured it wasn't likely to change in the town he lived in all his life, while under his "ghost scientist" dad's roof. This had led to a loner lifestyle that he can't seem to shake off in person, but he's done alright with some online friends. While he'd normally have Aradia and Sollux at his side at least, they're families hadn't moved to Amity Park, meaning that when the accident occurs, he goes through it alone.

2. Karkat is in denial much longer about his situation, but when it finally settles in when a ghost attacks, he's not exactly doing all that well. He cuts himself off from the world for a few days and eventually settles into his role as Phantom, though not without a great amount of bitterness. The one relationship he has, the one with Spades, sours and he kind of distances himself from his father when this starts to occur. This also starts a vow that he will make sure nobody knows about the whole thing, wanting it all to stay under wraps.

3. With a lot of maneuvering, he's able to make sure literally no one knows about his halfa status. Most of his ghost allies don't have any suspicions, and while it made certain things harder in some aspects, he's able to keep a balance. His ghost allies figure he's just a recently dead ghost that has some fascination with superheroes and generally play along to figure out more about his obsession, and what exactly happened to him to have things get him to where he was. Karkat is deathly afraid of anyone figuring out the truth since it would only cause more ghosts to be interested in him.

4. As for the human side of things, it's somehow even worse. Karkat has been doing his best to keep Dualscar at bay after he arrives into town with his son, knowing the man is dangerous and not to be trusted from the get-go. Kanaya and her family are put at arms length, even though he's very interested in learning about his family. His canonical friends move in eventually, Aradia and Sollux randomly interacting with him or Phantom at various moments. They cause the most headaches since they're very close to the truth, and Karkat doesn't want that either.

5. There's a lot of drama involved when Karkat can't help but wonder if being a ghost is actually better than being human, mostly because he actually has a large support system now, but only after he died. The secret does weigh on him greatly and it comes screaming out eventually, probably during the arc with the Muse, causing a lot of drama to be involved with the discovery.

The game is always open, so feel free to pitch some prompts any time. Again, I hope you enjoy!