Nothing that isn't completely and utterly necessary. I'd like to see them possibly add 1 or 2 new features to the game, but they shouldn't change any of the pre-existing mechanics from IGAU1. I just don't want them to make the same mistakes they did with MKX, which was attempting to fix something that wasn't broken. Instead they added armor, rampant plus frames, extremely unreasonable hard punishes, stupid neutral jump in punches, stamina meters, etc... The jump from MK9 to MKX warranted many unnecessary, detrimental changes to the series that it would have done perfectly fine without. If you change to many things about Injustice, just like MK, it starts to become another thing and lose its identity.
TLDR: If it ain't broken, don't fix it!! Maybe add a couple features but do not change preexisting ones