If there is one word that I could preach to Rockstar about the next installment, it would be this: customizability.
If that is even a word, it is very important. I need not repeat everything others have said here, but I would likely agree with a lot of things. For example, creating your own gang, house, business, etc. It would be vital and truly amazing.
However, most importantly, I would say they should add Parkour.
Here's a video of what I am talking about:
I don't know about everyone else, but when playing GTA 4 I really enjoyed running through the city and jumping over things, especially the mission "Ivan The Not So Terrible." I think if used correctly, Parkour would be truly perfect for a GTA game. Its been featured in Nike commercials and the movie Casino Royale. I think this sport was used in Mirror's Edge as well.
I would really love to advocate this for GTA 5 or whatever it will actually be called.
Whaddaya think?