I would like to have the ability to order food, like calling for cluckin' bell. I hope the delivery-guys would be regular, because I wish to avoid being as lonely as Carl was in SA. BTW, I would like Tommy or Carl to be the protagonists... Just joking, I know they wouldn't be. Also, if we'll have animals in GTA V, pet stores would do. I may post afterwards, should I have more ideas.
Edit: I've just got a few other ideas - you should be able to pinpoint a spot on the map, then use your own car to automatically drive there, without the need to call a cab to do this for you. This would also open the door for a teleportation cheat. And when you dine somewhere with someone, why not hear the protagonist and the fellows he dines with converse? And since it is a game for 18+, there shouldn't be any issue with pornography.