I apologise if some of my ideas are already in game or been done already. I don't have GTA V yet, but i'm looking mods already and posting now here my suggestions, just in case they aren't in game or no one has done these things yet.
Animals in city
To make city interesting especially when playing zombie mods it would be great to see animals like Cougars, Coyotes, Deers in city.
More Criminals at Night
When people in Los Santos are sleeping and there would less traffic it would attract more criminals.
Any car fire near the player would cause a firetruck to respond to the scene, investigate (look around, take pictures) and clean up (sweep) the area. The fire would also be extinguished. They would then return to a fire department unless there is another fire. http://www.gta4-mods.com/script/ambient-fire-dept-f8510.
Fixed Wanted Level
lvl 1 wanted level: You get ticket for minor crimes like in real life (http://www.gtainside.com/en/gta4/mods/60793-ticket-fixed/)
E.g. If cop sees you speeding, fighting with ped and ped stays alive. If you are shooting air with gun and get wanted and you let police catch you without resistance he take's your weapons and give ticket.
lvl 2 wanted level: If police sees you beating ped and ped dies or police sees you escaping while lvl 1 wanted level it turns to lvl 2 wanted. Also if police sees you shooting someone or ped reports you police you get instant lvl 2 wanted.
GTA TBOGT Drunk Feature
Feature if you drink too much and pass out and wake up morning in random places. I red that this is possible with Trevor but would be cool if you could do the same with other characters and also decide the possible spawning points.
Planes Crash Buildings
Sometimes planes hit in buildings like they used to GTA San Andreas (Somekind of .ini file where to adjust how frequantly they crash would be great of course).
Random Ambulance and Firetruck in street
Sometimes there would be Ambulance or Firetruck driving in street with their sirens on.
Realistic Car Weapon Damage
This kind of damage to cars: GTA 4 mod: http://www.gta4-mods.com/misc/bamanbois-realistic-car-weapon-damage-f30548.
So you can store weapons and ammo in safehouse stashes.