Random thoughts in no particualr order...
-I echo the Liberty City love, but only if it's the same layout as in 3. Sure make it bigger if you want, but keep the original streets the same. If it takes place in the 70's, then let the city be different(buildings not there yet, maybe the dam hasn't been built). Just by making all the buildings enterable(is that a word?) the game will already be MUCH bigger than 3.
-I don't think GTA 4 needs any online content at all. If they want to do the online thing, do a spinoff or something. An online GTA would just become an all out murderfest, no matter how much story it had . Well that's how I see it anyway.
-I too, would like much better graphics for the next GTA, but at the same time I'd much rather have more refinements. The poor textures and character models are two of the things holding back the current GTAs. At the same time though, I want more pedestrians(well, where there should be pedestrians), more traffic(especially on the highways, have a rush hour even). Also instead of making a billion new things to do, refine what's there first. THEN add more things to do. Also, the game should keep track of the last 10 or so vehicles you used, would be very helpful.
-I think with the customization we saw with San Andreas, we can expect to be able to choose from 3 or 4 characters for the new game. White guy, black guy, woman, and a mute(GTA 3 guy maybe?). I just threw in the mute for people who get sick of the random quips(I do from time to time). If their lines are different(or even different missions), it will give a good reason to play through again. Or maybe you could select a certain character for certain missions and have them all to choose from at any time. Like an A-Team of sorts. If you want, you can bring them all and do missions together. Either computer controlled guys following you around(as long as they're smarter than the ones in SA), or have the option of 3 friends helping you out in certain missions(with the difficulty ramped up to make it work).
-In some ways GTA is already cartoony as hell, but would it really hurt that much to have a Halo-like "flip car" option? Or something else? And does every flipped car HAVE to explode? This isn't "TJ Hooker" people.
-I guess a walkman would be okay, but I really enjoy running around just listening to the world. Still, I suppose the option should be there for people who want it. It should have drawbacks though, like maybe not being able to hear gunshots or something.
Anyways, there are my jumbled thoughts on the matter. The game doesn't need to be as big as San Andreas, but I'm sure Rockstar will go bigger, otherwise it would seem like a step backward to some.
PS- I too would love a Grand Theft Horse(San Andreas in the old west days?), or Grand Theft Auto: Apocalypse(choose your flavor, but zombies are my choice).