For the multiplayer I would prefer it if they got rid of the deathmatch and mafia work type modes completely. I think free roam should stay and you should be allowed to do everything you can do in single player. In IV simple things like there being no trains and not being able to play darts and bowling with your real mates in freeroam really dissapointed me. So, more fun things to do like fly planes and be able go inside more buildings would be great.
Also I think V should have a whole new level of customisation. These should be a much wider choice of clothes, vehicles and weapons. And I really hope this time the map isnt just one big city. I want to have access to countryside and mountains like SA.
Oh and more hidden things for us to find please