more cars... obviously, maybe put a respawn time on them or something so u dont have an empty lot at spawn
activate stunts... not neccesarly for the money... but to see what stunts you are doing. this probably would be client side... no harm done to bandwidth.
activate amunation and garages and such. this also would be client side. im supposing.
adding a console or some time of thing to look back at what people have said... possibly one that scrolls.
a stats page.. much like counter-strike or other half life... with player name. and the number of the person (read in previous post that changes the radar blip to a number) and ammount of kills/deaths... ping might consume bandwidth... dont know how half-life relays that :\
a setting to allow cheats... or atleast allow the car cheats incase u get stranded without one.. just make it so theres a timer so u dont spam piles of cars...
the multiplicity thing is a great idea... keeping it so singleplayer is not interfeard(spelling).
some other stuff ill reply if i think of any more
those are the things i'd like to see happen to this in later versions of mta, if not early.