I'd personally like better facial expressions (IV did it good, not great, but good), more possibility to decide how the story goes and especially more gadgets/weapons (Night vision, jetpack's, parachutes, grapling hooks, bombs, etc.), SA & TBoGT did this great.
Also more realistic crashes
and effects will be good for GTA instead of the windshield thing, like with a high speed crash that you will see the parts of your car (Witch you've borrowed without asking for permission) flying around.
+Better AI of your enemies will make the game a lot more interesting, because in IV and other GTA's they didn't even tried to surrender you when you're on foot, don't even mind that they're trying to hit or pit you with a car.
+More ways to fight will be cool, like trowing enemies on the ground or hitting them with stuff you find, like a pipe as a bat or swinging a shopping bag against something,
+Missions on public places were you don't have any firearms and more specific sniping missions.
+Higher jumps, that little jump of IV is even the worst of all.
And the most important: A better PC port.