Make it more real !
* Players being capable to blow up everything around them. For example, fire and RPG rocket on a building, part of the building to blow up. And some how after you blow up the building the Town Hall should send workers to undo what you did, and rebuild the building, and it sold take like 7-8 days depending on the building's damage.
* Make an online version, like a online community, no bots, just real players, all the positions, form medics to police officer to be filed by real players. You should be capable to buy you own house, own your own company or work at one, buy you car, be able to lock it. On this section there are numerous things that can be done.
* Be able to create your own character. Make it black, white, Chinese, Japanese, what do you want.
* I don't think there should be more missions in game, there should be a better AI, for example you can choose for a large number of carers. Lets say i want to be a member of a gang, i go and join a gang, first i will have a lower rank, maybe selling drugs at a corner, every day you will have to pass different obstacles on a daily basis.
* Be capable to interact more with the pedestrians, form a family, have kids ? ( Don't think this would be possible, but why not ).
* Be capable to do almost everything that is possible to do in real life. From paying your rend or electricity bill to running a crime organization.
* The map should be very big, with lots of new things, lost of new cars, planes, trains, bikes. Places to explore and to find new and very rare things, have animals in game, from dogs, to elephants at the zoo, and to be able to find rare animals.
* I would really like to see in GTA, some how to play soccer, or ride a bike in a freestyle mod. For example, you hop on an bike, and you should pres a specific button, when you press that button the key boards or the joystick buttons would change for you to be able to perform tricks and stuff like that, and that's the same with soccer. Like for example in car commands and on foot, they are different.
There are so many thing to add, these are just the ideas that came to me right now, but there are so many things that can be done.
I'm sorry if my English is a little rubbish !