Some of these might have already been suggested but here's my small contribution:
- As you work your way up the wealth ladder, you get the ability to build your own house from scratck and choose the finishes.
spend your money buying really big mansions with huge yards and not the bachelor pads we've seen.
- Ability to throw parties at your mansions - you can invite all contacts on your phone - they in turn, can invite their friends, etc.
- Large cruise ship with passengers (the wealthy types) on-board. One of the missions could be to rob the ship of millions of jewellery or something like that. After the mission you are free to explore the ship and even take if for a spin. The ship populace should always be there before and after the mission.
- Special mission: Fly to the moon/mars - don't know what happens once you land but Rockstar can think of something I know.
- Bring back girlfriends ala GTA: SA
- Go back to one protagonist and expand the story, however, maintain the importance of other protagonists in influencing the final outcome.
- Bigger, better, explorable airports.
- Prison missions - as in like you are in jail and have to do everything to survive.
- Now that we have the stock market can we have the ability to manipulate the moeny market, if you get caught you cough up heavily or even have your assets attached.
- More usable stadium/areans in the game. ability to watch games such as basketball, base ball, soccer, boxing etc.
- Dynamic weather, yes. You have to dress accordingly. If it's snowy and you're under dressed you die, just like you did if you didn't eat in GTA: SA
- High profile missions like infiltrate the presidential palace and steal state documents or kidnap the pres's kid.
- Malls, malls, malls, explorable ones.
- Big-ass submarine with nuclear bomb intended for use in the city approaching... you have to help state/military to prevent this from happening.
- Own a recording studio where you sign artists - actual music recorded by your artists is played on the in-game radio stations... the more popular the artists, the more money you make.