I mean if I wanna go super selfish and unrealistic I'll go for it.
Move Tutors, Battle Frontier or PWT, more cameos of older characters in the Battle Tree, a bigger look at the backstories of Lusamine, Mohn, Guzma, Nanu, Looker, Anabel and others, more appearances for Gladion and flesh his character out a bit more, the Ultra Beast having a bigger role in the story, Difficulty Settings, expanded Trainer Customization, new Alola Forms, new Mega Evolutions, a tougher Elite 4 rematch and give all title defense challengers 6 Pokemon, new Totem Pokemon and Trials, Kahuna and Trial Captain rematches post-game, and I think that's it. Will most of this happen? Hell no, but it would make USUM perfect for me.
butpleaseGFfortheloveofGODgiveusaBattleFrontier