Better mansion definitely ... at least one with a garage.
I like the idea about the interwoven, separate storylines. How about this: you can choose between a number of characters; a white male, white female, black male, black female. Now what if each character spawns a different storyline? Just an idea.
I really, really want a different speed variation system installed. The traffic on GTA isn't set to coincide with your speed, but every car travels at the standard fastest speed of most of the normal cars. I wish each area had a speed limit sign somewhere, and cars abided by this. I also wish your car had a speedometer, and possibly even a button to press which you could activate a speed limiter which automatically abides by the set speed limit of each area. Mafia was on the right path.
Blinkers? Mafia had this set, too. Another thing that Mafia had in it was what was mentioned before, that doing menial things (1 star) would warrant a ticket symbol on the top of the screen. You could choose to pull over and receive a ticket, but you could disobey this and head on a chase. This would warrant a handcuffs symbol, where the police would chase after you, and handcuff you when they had the chance. I wish handcuffs and tickets were input into GTA. If you killed someone, or shot a gun at someone, the police would go into a sort-of "take down mode" where they would take you out (shooting, running you over, etc) at any cost. This would be great in GTA, and make for some fun for people looke for it, and make it easier on those wishing to abide by the law.
More realistic health deterioration system where, say, getting run over would take off more health than it does, or falling off a bike. The impact of the hit and fall could determine the health taken off. In fact, Mafia had a system where running into things with your car hard enough would result in health loss. I doubt we would see this in GTA, but it would make for better drivers. Possibly ability to toggle some of these things on and off? I doubt it, but you never know...
It irritates me when the CPU controlled traffic lags and don't drive smoothly. I realise this was a result of overloading the disc, but hopefully it will be gone. Possibly make lane-changing by traffic slower and smoother. Smarter traffic, which looks before turning into a corner rather than merely driving into it and hitting you would be nice.
GTA clock set to your Playstation clock, rather than advanced time? I really want this, and have done for a long time.
If, and most likely when, more cars are added, input a feature where cars have brands and makes. Like a Chevrolet Washington, and Chevrolet something else, but obviously they wouldn't use real car brands. Year models?
How about not blocking off each island/city? I know it gives the incentive of completing missions, but possibly rid this, allowing full access, but give the incentive of unlocking nice, one-of-a-kind cars, big houses/mansions, entry to certain smaller areas, etc? I'm starting to get annoyed that there has to be a reason for the closure. Only GTA III pulled it off properly. San Andreas only gave a hint of the reason (CJ on parole), but blocking off each city wasn't warranted, they should have just left the wanted level part in.
In-car and first-person view? Please, please, please?!
Most things are fine, and improvements on things like graphics will be definitely expected, and included, though I don't mind the "GTA-style graphics", just improvements on little glitches would be nice. Saves space for additions to the game, too.
Nice idea though! Maybe there's a mod for a current game out there that does that ....
) or health boost as in Vice City. What a rip-off 
. I`d also like to see some kind of reward for a high criminal rating, otherwise it`s pointless except for bragging rights. What the rewards would be, I don`t know. That`s for R* to decide, I guess. 
