I'd like to be able to upload and download other people's custom fighters like in Fight Night Champion. The photo game face in EA UFC was pretty poor. I used to beat the shit out of Butterbean with the 1974 version of Schwarzenegger and the settings tuned to cripple.
Maybe add PRIDE, Strikeforce, Affliction, and WEC PPV arenas similar to what they do with the WWE games. Classic skins too. And a story mode that takes you from the early UFCs right up to UFC 200 (or the closest PPV to the game's release date).
But to be honest, I'd rather have a new Fight Night.