To futher add to what I said, for any future GTA I expect certain things from the game.
i personally advocate the use of realism for the mechanics for the game.
Physics :I think most people can agree that Euphoria/RAGE engine is superior to what they used in SA "RenderWare". The way bodies accurately behave to the whims of physics, and how a hopeless person flies through the air when I smash into them with my car that has my grill bent inwards with blood all over it, just spells GTA. In future GTA they should use the updated Euphoria like in Red Dead. A lot of people are pissed off as Niko takes his sweet time walking up/down stairs at a realistic pace. However in Red Dead Marston gets up those stairs 3 at a time, not as annoying.
Driving: The driving, now I know a lot of people don't like the driving. Reading the coverage before the game came out stating the driving would become more realistic didn't prepare me for what I was about to experience. I think everyone's first driving experience in GTA IV was the same. Gas, brake, brake, brake, smash, smash. Anyone though willing to spend the time to learn the driving was probably pleased later on. When I play multiplayer and I'm hugging the roads and not smashing, I feel like I actually have a legit skill. It feels a lot better to drive with these more realistic driving physics, then in previous GTAs where you can accelerate 0-60 in about 4 seconds in everything short of a transport truck, and stay on the road no matter how big the curve is. Every time I went back to play GTA for PSP, SA, or saints row I felt the driving wasn't right. I felt like it was too easy, and just didn't seem as immerse as GTA IV.
Detail: Now I know a lot of fans don't like the new realism but I would find hard to believe anyone hates the detail in the game. This game has a insane amount of detail in it. I would actually walk around to soak up all the stuff Rockstar has put into the game. Shear awesome. GTA has always had some detail in it, but GTA IV put quite a bit of it in. All future GTA should have such detail.
The future: Now with those underlying mechanics in place I think GTA should continue what it does best, out of this world characters, more side missions, customization, and just some crazy things. Like all the cheat codes? I'm not 'cheater' but once a game is done, i like to put some of these cheats in. I like angry mobs of citizens chasing me. But of course I can only do that in SA. I do miss SA, but I hope one day they remake SA like they remade liberty city. A huge map, realistic physics, crazy stunts like breaking into military bases, you know that stuff we love. But most of all I like the crazy amount of things you can do in SA. I mean, you can be a train driver, do triathlons, eat 11 buckets of chicken in a row, pimp your car out. Its all things that should be in the next GTA.