Let's say that hypothetically we have a new universe that brings together the most realistic (and best) stories of GTA: IV and Vice City. What would you change to make San Andreas a better and more realistic story?
Why would there be SA in the universe that "brings IV and VC" stories together?
Anyway, I would have more missions for Toreno and Truth (re-instated mission where you need to save Truth too). Possibly, more Catalina - she's kind of hot, but also all of these characters' missions make a big use of the countryside and the desert, which are my favourite areas. I would definitely want to do some spy/alien stuff for Toreno and wacky bigfoot/conspiracy missions for Truth.
Other than that SA is pretty good; the only improvement that would really make a difference - another area/city with new characters.