I expect they'll be taking a lot of inspiration from the popular features in RDR. Gang hideouts, gambling, interactive free roam, social club challenges, etc. I think some of the picky features everyone asks for before a new GTA game (petrol, ability to do x, a mission where you are able to do y, etc) are going to be ignored in favour of 'social' features, challenges, leaderboards, and suitable rewards (avatar awards in RDR were cool. Shame there were only nine, but they were probably testing the water).
Will be interesting to see how they handle outfits - will they stick to GTA IVs idea of clothes shops and a few special outfits you can unlock, or go with RDRs model of unlocking outfits as you complete a series of tasks? I think the shopping model applies to GTA IV better but RDRs outfit selection was MUCH more varied and interesting. In GTA IV I found myself using the same two outfits, where as in RDR I use a number of different ones. The selection was extremely limited in BOGT and LAD didn't bother at all. It will also be interesting to see if they give some outfits different bonuses too - but I suppose that all depends if Rockstar is going to go for realism again, or more of a relaxed/fun atmosphere. I guess RDR could get away with it more easily since the Western setting has an aura of fantasy about it. Maybe Rockstar will let us vary the outfits better by letting us choose our own colours or something? Who knows, I just hope we can have a larger number of decent outfits than GTA IV.
One thing I hope Rockstar does is make it less of a grind to progress in multiplayer. I do not want to have to grind to rank up and unlock stuff! Let me unlock guns/outfits by completing challenges if you want, but I don't want to have to play for 100s of hours to unlock everything I want to own. Maybe let people choose what they want to unlock at each rank rather than have a set order. The game is fundamentally a sandbox game, so there should be fewer restrictions than you get with a normal FPS game.
As far as the actual story goes, I'm open to ideas. America has the diversity you need so I expect GTA V will be set there. I'm more interested in having a better range of cars with better sounds and handling (GTA4 was an improvement, but you had Infernuses with jelly suspension), a good selection of guns (again with better sounds), and I'm also hoping for some more graphic gunplay. Sure you could get clouds of blood in GTA IV, but there was no dismemberment or anything like that. Improvements to the cover system are pretty much guaranteed so I look forward to that, too.