Ah, where to begin?
I won't go into much detail, but I definitely want to see a vast improvement from GTA IV.
Many types of aircraft
Vehicle modification shops
More player customization (more shops, more clothing pick-ups/awards, etc.)
Good music and entertaining talk shows on the radio, television shows
Badass weaponry
Activities: going out with friends, having girlfriends, etc.
A lot more accessible buildings
Many Easter eggs and (funny) glitches
Multiplayer: a lot more online game modes, more customization, perhaps an in-game chat?
Customizable radio station (for consoles)
Ability to replay missions
A LOT of Rockstar humor
I believe that's about it, if I appear to be missing something, I shall add it!