Yes but someone already started doing GTA V MP Mod don't know how.
http://v-multiplayer.org
GTA V is made with same engine and same scripting language as Max Payne 3 and people who doing this project http://v-multiplayer.org said that Max Payne 3 was very easy to script not as GTA IV was.
Also other team is doing something http://www.vmultiplayer.net/
So i think we can have some sort of MP Mod on GTA V soon after PC release.