Batman & Katekyo Hitman Reborn
Dick and Skull were raised in the circus together. Skull's mom as the fortune teller and his dad the stunt biker, hence his obsession with motorbikes. Dick and his parents are the acrobats obviously. Skull is a couple years older than Dick and treated him like a younger brother.
When the Graysons die and Dick is adopted by Bruce, Skull is the only person Dick kept in contact with from the circus. Bruce and Alfred know because they arranged for both boys to have a way to communicate. However when the other kids get adopted, none of them have any clue about Skull. This started because Dick at first didn't know how to open up to Jason and wanted to keep Skull private since he was the only piece of his old life left. But Dick does try to be a good big brother to Jason, since he has Skull as a healthy support system. He argues less with Bruce because he's able to vent to Skull and Skull helps him sort out his thoughts. Skull knows about Batman and Robin because Dick and he made a pact as toddlers not to keep secrets from each other.
Then Jason got blown up and Bruce went off the deep end. After an argument with Bruce, Dick leaves the manor. He moves to Bludhaven and Skull comes to visit him for the first time in person since they were separated. Skull stays for several weeks before Tiny Tim turns up on Dick's doorstep. Tim tells them about how Batman is out of control and how he figured out Dick's identity. Skull is in the other room so Tim doesn't realize he's there until Skull bursts out laughing when Tim explains how he figured out their vigilante identities. Tim is startled when Skull joins them and freaks out thinking he revealed Dick's secret identity. Tim starts to spiral and panic, which sets off alarm bells in Skull and Dick's brains. Skull goes into his big brother mode and calms Tim down while Dick starts packing a bag.
After reassuring Tim, Skull says Dick is going to take Tim home. Dick adds on by saying he'll do something about Bruce. Skull says he'll stick around for a few more days in case Dick needs help. Dick ends up knocking some sense into Bruce (after a very brutal "spar" between Nightwing and Batman) so Skull's help isn't needed. Skull locks up the apartment and heads off to the location for his next stunt show.
They keep in their usual contact for the next few years, while Tim settles into his role as Robin, Dick alternating between Bludhaven and Gotham, and Bruce learning to be a more emotionally stable parent. Skull visits Dick occasionally at Bludhaven, if he's nearby, but otherwise they just do video calls and texting.
When Jason comes back from the dead and tries to kill Tim, Skull shows up and helps Dick capture Jason. Jason is left in Skull's care for "rehabilitation therapy" which is basically Skull giving Jason affection and counteracting the brainwashing Talia did to him. Jason is still the anti-hero Red Hood, but he's no longer murderous to the bats on sight. He's our favorite grumpy Jason again.
Shortly after, Skull gets invited to join the strongest seven, which he thinks is super suspicious because "it's like a rip off justice league" but decides to check it out so he can report what he discovers back to Dick and Bruce if it's actually something. He doesn't expect to get mixed up in the mafia but he's pretty chill about it considering his best friend is a vigilante.
One day he gets a call from Dick while with the other members of the arcobaleno. Dick is panicking because Damian has arrived and Bruce is missing and presumed dead so it's up to him to handle the tiny stabby child but he has no idea how to be a parent and he needs advice ASAP. Skull talks Dick out of the panic and gives him some advice. He promises to send parenting books in his next care package. Skull scolds Dick for not telling him about Bruce sooner and Dick tries to say he didn't want to bother Skull who was preparing for his show, but that just makes Skull scold Dick harder. Finally Skull wears himself out and starts offering advice about what to do at the current moment. Because of this, Dick handles the whole Tim and Damian situation much differently. Tim chooses to pass Robin to Damian and leave to search for Bruce, but completely by his own choice. Dick supports him and offers to help in any way he can, but Tim tells him to take care of Gotham and that he will keep Dick updated.
Skull runs into Tim during his travels and gets dragged into the whole Ra's thing. With the assistance of Skull, Tim isn't injured as severely and Owens and Z survive as well, because I love the idea of Tim just having a trio of assassins loyal to him. The five of them destroy the League of assassins base. Skull and the assassins part ways with Tim. Tim takes Tam back to Gotham and takes over as CEO. While that is going on, Skull and the assassins bring Bruce back from the time stream. Bruce is weakened and since the four of them don't want him to know who saved him, they keep him tied up and blindfolded. Skull trusts the assassins to return Bruce to Gotham and suggests that they stick around if they want to. He says Tim could use more friends his age. Pru, Owens, and Z don't say anything but they promise to think about it.
Skull returns to the Giglio Nero manor and life returns to some semblance of normal. Or as normal as it can be all things considering. Then the curse happens. Skull doesn't know what to do now that he's stuck in the body of a toddler. He goes to Dick, hoping that maybe he could get in touch with a magic user from the Justice League and break the curse. Dick is needless to say, shocked at his best friend suddenly in the body of a toddler. Skull moves in with the Batman while they try to break the curse on him.
I don't want to wait for canon for them to creak the curse, but it's too easy if the JL is able to break it, so I'm thinking a compromise. Skull is still cursed, with the pacifier siphoning his flames constantly, but they are able to get him his adult body back. Skull asks if it's possible to do the same for the others that were cursed alongside him, and tracks the other Arcobaleno down.
They are eager to regain their adult forms, even if they won't age and still have the curse on them. It's better than being a toddler. Checkerface is pissed at this development but Skull threatens him back, allowing his cloud rage to show for the first time. Checkerface backs off after that.
I don't have anything else planned for this idea, other than some slice of life scenes of Skull living with the Batman and interacting with the others. I see Bruce kind of adopting him into the brood without issue, and being grumpy when he learns it was Skull who got him out of the time stream. Alfred loves having someone else around that can wrangle all the chaotic ones, even though Skull has his own moments of being just as bad, if not worse. Dick is ecstatic to have his BFF with him constantly. Jason doesn't even try to fight it, since he vividly remembers the time Skull had him captured before. Tim enjoys having Skull around and even asks Skull to teach him about various stunts. Damian takes some time to warm up to him but this is Skull we're talking about. Skull wins him over in the end and makes him a less murderous baby. Skull could even be the one who gets Damian his first pet and starts the obsession. Cass and Skull have a great relationship as well, with Skull doting on the girl and helping her be more confident in public. He knows when she needs to shine and when she needs people to take their attention off her. Stephanie bonds with Skull over their love of purple. Skull helps her with becoming even more chaotic. Skull and Babs bond over computer stuff. Babs having started teaching Skull while he was cursed.
Skull could join the nightly activities alongside the others and be a stuntman during the daytime. I also want to include a realization of Skull's immortality thing, because I love the trope of people thinking Skull is dead only for him to get up without a scratch after a few minutes. Bonus points if Skull didn't even know he was immortal until that moment though.
That's about all I have for this idea. It's a little scattered but that's how my brain tends to work when planning my stories. Hope you all like the idea! I actually have a couple more with Skull and Dick knowing each other, that I'll probably post soon.