Looks very promising, great job. I also like the avengers hyper added to Cap. I have also thought that Falcon and Winter Soldier when make a nice hyper as well, maybe even have them all in Captain uniform.
Mimic, or Exile's Mimic would be cool. Normal Mimic has the powers of the original five X-men in addition to being able to copy other mutant's powers. Exile Mimic retains the power of Colossus, Cyclops, Wolverine, and Beast. Good luck with your choice.
Re: Beast by Gladiacloud, MotorRoach, and Kenshiro99
These stages are very well put together. The night version is so cool. These stages definitely put you in the mood for a good Mugen Spider-Man game lol.