1) I'd like them to remove the stage transition moves. They suck.. kinda cool the 1st few times you saw them but they got old FAST.
2) I'd also like the "clash" mechanism completely removed, possible replaced with something else. It was a total failure on all counts as a fighting game system. OMG.. get rid of it.
3) The interactables in injustice were a complete failure as well. MKx was a huge step forward in that regard. I'd rather not have them at all if they are as lame as last time, but if they follow in MKx's footsteps style, then I hope they are in.
4) On a very personal note I would like Lobo completely redesigned. He is one of my favourite DC characters and I really didn't like his injustice version. His moves, his walk animations.. nothing. Single most disappointing thing in the game. So put Lobo back in and this time make him awesome. Oh and if you use the 52version of Lobo I WILL hunt you down.
5) Besides the standard JLA and DC Trinity I want to see Lobo, Zantana, Sandman, John Constantine and Swamp Thing.