When it comes to my choice of guest character, I've got one word to say: Cowabunga!
The Teenage Mutant Ninja Turtles
These Heroes in a Half Shell are no strangers to comic book crossovers - they've teamed up with Batman several times in recent years, stopping the Shredder from taking over Gotham, stopping Bane from taking over New York City, or stopping Krang from remaking the multiverse in his own image. (those crossovers escalated quickly)
I'm sure most of us know who the Teenage Mutant Ninja Turtles are, but for those who don't, after four ordinary turtles were mutated by radioactive ooze, Splinter taught them to be ninja teens, and named them after various Renaissance artists. They originated in comic books by Kevin Eastman and Peter Laird, and they're green, but not rookies by any stretch of the imagination. They've built up quite a rogues gallery over the years - when the evil Shredder attacks, these turtle boys don't cut him no slack, but they've also dealt with the alien warlord Krang, the bumbling Rocksteady and Bebop, the rock-man General Traag, and the mutated assassin Tiger Claw, among others.
However, there's four of them, but only one spot. On one hand, Leonardo leads, making him the natural choice, but on the other hand, Donatello does machines, giving him more moveset potential. Raphael is cool but rude, which seems like it would make for more entertaining dialogue, while Michelangelo is a party dude. (PARTAAAAAYYY!!)
As they're the world's most fearsome fighting team, I think we can find a way around it. My suggestion is to have their Unique Ability be to switch between the four turtles in the order of the theme song (Leo --> Don --> Raph --> Mikey --> Leo), with the speed, range, and power of their attacks being different depending on which one you're using.
Likewise, for their costumes, I'm thinking that we could have each turtle serve as a separate costume, like so:
1. Leonardo
2. Donatello
3. Raphael
4. Michelangelo
5. Leo's alternate costume
6. Don's alternate costume
7. Raph's alternate costume
8. Mikey's alternate costume
They'd technically have less costumes than everyone else in this set-up (with each turtle having two costumes), but this way, people can pick their favourite turtle to start as, since we don't have Injustice's gear system. However, that's just my own take on it, and we can decide on that (if they get in) when we get to the costume job for them.
The Teenage Mutant Ninja Turtles have come a long way from starting off as something of a Daredevil parody (Stick --> Splinter, Hand ninjas --> Foot Clan, etc.), and if we're including guest characters in here, I figure that these brothers should get a chance to give the worlds of Marvel and DC a taste of Turtle Power!
(out of the lyrics to the theme song, it was tough to fit "and they're green" in this description)