Personally, San Andreas was my favourite GTA Game to date. Yeah, call me generic, but whatever.
I would like to see more customization like in San Andreas, from haircuts, tattoos and clothes, to fat, muscle, stamina, sex appeal etc... Also featuring RDR physics, those little details such as visible bullet/stab wounds, diverse hit detection (e.g, if you shoot them in both legs, they can't move). I would also like to see more random events, such as the cannibals and such in RDR. Going to the gym to stay fit and learning to fight, gamboling, eating and all these activities are what make the game so fun. Having hundreds of different pedestrians with AI incorporating some sort of free-will would be AMAZING. So not only would everything be centered around you, rather Npc v Npc.
A San Andreas like game constructed using the RAGE Engine would be what I am hoping for. Also in San Andreas there are so many different environments, there's the city life, the country side, the forest, the beach... And there were SO many easter eggs and fun cheats (I only play with cheats after I've beaten the game's storyline).
Another thing that I really want is a believable character that I can connect with, like John Marston, my favourite videogame character followed by CJ and Ezio.
I also love Hip-Hop, and even though that was incorporated in the original San Andreas I would love hip-hop culture to return to the game, such as rap, grafitti, DJing and B-boying.
This may make me seem sadistic and evil, but I LOVE killing helpless pedestrians in game... A more flexible violence system, such as being able to severe certain limbs (not too graphic, obviously) would add to the replay value of the game, such as hitting someone with a car and actually seeing their bones break.
Maybe I am asking for A LOT, but I am confident Rockstar will be able to deliver. Regardless, this will be a fantastic game, knowing R*!
EDIT:
I would REALLY love to see a drug dealing aspect, like in CTW, I became a millionare by knowing the right people.And maybe a more advanced theiving/robbery system! I would love to be able to pick-pocket, B&E houses, rob stores (Food, clothes, car etc...) and be able to ambush drug deals and take the dope and the money. Also I agree with one of the previous posts, you shouldn't instantly gain wanted stars on murder. I would love to be able to hide in a dark alley, ambush someone, then put their body in a car and dump it anywhere. That would be hilarious!