Most of it to be honest.
I'd like a more balanced economy (which is unlikely to happen even in GT8, but I can hope)
The single race format replaced with more championships of varying lengths
Re-winnable prize cars for winning race series and championships
Ditch the GT cafe
Re-work the invitations to buy cars into a manufacturer relationship system where buying and/or racing cars of that brand increase the relationship to the point they'll sell you their special invitation only cars
All races to be standing or rolling grid starts, only use stretched out chase the rabbit races for special challenges/scenarios
Custom championships
The UCD and LCD to be tied to in game days, not real time days, so the lineups rotate as you play, not as you wait
There's more. We're probably opposites in how we play, I don't touch online.
FTAOD I don't hate GT7, it's just not as good as I'd hoped it would be, for me. I can't criticise the support the game has received since launch.