Next gen GTA wishlist:
Enormous map with multiple cities/towns/districts
Always online gameplay with dynamic changes and persistent world
A player-controlled economy
Massively improved NPC AI
...take those four main points and add all of the little details and you basically have the best game ever made.
By taking development resources away from SP you can make a truly intense and crowd pleasing experience that will appeal to every play style. PvPers and Mission monkeys will both have their needs satisfied.
The only ones that will cry will be the "I never play online" cockholsters but those guys are becoming increasingly irrelevant and soon will never be taken seriously again.
I have high hopes for GTA Online but I think the real revolution will be with GTA 6 if they do it right.