So, This is a bit of an obvious one in my opinion, much like the last one, I suppose.
Anyways, Ben 10 has a very big Fandom and is very popular, don't know about 3Below that much, either the Fanbase is smaller, or not as eccentric, something that isn't hard to be honest, but still with the summaries anyways.
Ben 10, in short, is a show about a the life of a kid that has had an alien watch on him since he was ten. The watch, originally, allowed him to turn into ten Aliens. Being a ten year old, he followed every kid's dream and became a superhero, hence the first series was born. Ben; his cousin, Gwen; and their grandpa, Max, are on a summer road trip. The two cousins hate each other, obviously. From there, a bunch of stuff happens, Fight a bunch of Villains, An Squid Warlord, a kid named Kevin that can absorbs things leading to him becoming unhinged due to absorbing the Alien DNA in Ben's watch, and many more. Blah, Blah, Blah, A bunch of stuff happens, and the final episode where summer vacation ends is decided to be not Canon, in favor of Keeping all secrets in the dark for the next. Ben takes off the watch eventually, puts it back on when Max goes missing, Forms a team with Gwen and a now Sane and redeemed Kevin, Fight a race of aliens, named DNAliens, that basically transform other organisms into their kind through small brain squids..., Fights the Squid Lord again after breaking his watch, Fights an evil alien with his own watch who gets stuck as an Albino version of Ben wearing red, Blows up his watch before getting a new, weaponized one from his evil twin, fights squid Lord again, fights another member of Kevin's species, Kevin goes insane from absorbing approximately 7 times as many aliens as he did in the past, Ben becomes super dark and almost kills him but cures him by his Cousin's insistence, they're friends again! Then they get wrapped in a fight between medieval Knights and a Squid Cult, before they fight a new, interdimensional squid lord that used to be a dragon lord. The original squid lord, absorbs the new, more powerful squid lord, and becomes Squid God, Ben defeats him, gets a new, non-weaponized watch, Kevin follows Gwen, now his girlfriend, to College, Ben gets a new alien partner with a classic Rule following cop attitude, They fight a bounty hunter with an animal version of Ben's watch and an evil clock alien through time. They then go on vacation!
3Below is a show about two royal siblings escaping their home planet which is being taken over by a tyrant. and coming to Earth. They get wrapped into school, make friends, learn about human culture, and eventually free their home planet while dealing with high school romance, music, bounty hunters, definitely not Area 51, and much more. There's also a bit a lot similar to how Transformers had Unicron secretly be Earth, and such, but that's not important.
Not the best Summary, and My grammar is probably terrible right now, just started typing on a computer recently, but the point is, aliens! pretty obvious, right.
For plot, I was thinking that maybe The Plumbers got unknown alien readings from Arcadia, and send Ben and Rook to the the small town, where they meet the siblings. Hilarity ensues, where maybe they think Ben and Rook are bounty hunters or something, and fight. In the end, they work things out. Then maybe Kyber shows up, having been one of the bounty hunters hired to get the royals, only t find Ben with them. Maybe kyber could have gotten the Nemetrix Back and his new pet too. Some other stuff could happen too, but I don't recommend doing the two big villains team up to get rid of neutral enemies as that is heavily done in many oneshot crossovers.
Anyways, that's all I have for this one. Au Revoir.
PS: I'm learning French! It's Fun, in my opinion.