I'd also like an Injustice 3 that ties up the story and ends it in a more hopeful place. Maybe some time-travel shenanigans to go back and "undo" everything so that Joker never made Superman kill Lois and the whole "Superman going Evil" thing "never happened".
You'd be amazed just how many people purposely ignore and have never played those two truly awesome "Injustice" games, simply because they refuse to accept the storyline. They're like "I don't care if it's the best fighting game ever made; Evil Superman, I'm not touching it."
I personally love the series, but I feel like there's still some story left to tell AND that if they manage to tie it off as a nice little trilogy with a "Happily Ever After" to boot, that might be something that can satisfy a lot of people.
Regime Superman died pretty definitively at the end of the "Masters of the Universe" cross-over, but I don't quite think those comics are canon.
So they could still do one more if they wanted to.