What I would like to see in GTA5 is:
1) More Stores & places that you can go inside of when not on a mission. (More Interactive environment)
2) Ability to pick what color your car gets painted when you visit the 'pay-n-spray' shop.
3) Able to use taxi to pick-up & drop-off people to earn extra cash.
4) Have a garage at the house to save more cars/vehicles.
5) Able to hook up trailers and tow them around.
If they can add these things and keep everything that they did in GTA4, this game would be even more awesome!!!