since you could drink in drive in gta 4 i hope in gta 5 you could buy and smoke weed. you would have to actually find a dealer, get his number and call him to buy. they can have lil mini missions where when you buy its a set up and you have to lose your stars. 

in gta 4 when your drunk the screen shakes and you cant stand still. in gta 5 when your stoned colors are bright, things around you are weird and depending on the weed you drive really crazy. 

just my thoughts, cant wait for gta 5