My suggestions:
--Make liberty city,vice city and san andreas in one game, I know it would take a lot of space
--Add more cloth shops and get back those barber shops, a lot of safehouses, body statistics,gang war etc.
--make it as realistic as possible
--let the protagonist join his own gang
--access maybe about 50% of the total buildings
I am sorry if I repeated other suggestions it's just because I didn't read them
that's all