The whole teenage aspect comes from DC Comics’ Teen Titans. The mutant part came from Marvel Comics’ X-Men and the ninja part came courtesy of Marvel’s Daredevil. The turtles part came out of thin air.
Capcom tends to get shafted in these final boss fights. They've had some memorable encounters - I'm kind of holding out hope that Astaroth from Ghosts N' Goblins sneaks in, but I'd be a little surprised if he did. He's one of the original "huge" bosses, and he's always had a fairly interesting design that would work with MvC3's art direction.
I BRING GREAT NEWS - go to my blog to see 4 new character confirmations (Dante, Felicia, Capt. America and DEADPOOL), new gameplay details and a breakdown of the screens first released by Game Informer!