They're far-fetched, but:
Jetpack
Zombie Mode
First-Person Mode
No Cops
Spawn List (Vehicles)
Spawn List (Weapons)
Health
Armour
Ammo
Super Jump
+$10,000
Ped Riot
People Hate You
Parachute
Super Punch
Huge Explosions
Telekinesis (Not gonna happen, I know.)
GTA III- Era graphics mode
Overhead Camera (Like the first GTAs)
I know alot of these probably won't happen, but there's a chance.