dual pistols and uzis would be a great idea, but only with selected low-level weapons and you wouldnt be able to run/walk as fast,and you would have increased accuracy.
my idea, being able to shoot on the ground at all times - so you can jump and shoot, crouch move and shoot and the likes. Maybe make the aiming as accurate as an fps. also, maybe being able to walk a little while aiming with the RMB.
an fps view would be cool as well.
front mounted machine guns - not a fan of them, as its not all too realistic, and we can drive-by etc already.
also - the one thing i really wish gta would allow is for you to replay missions once you have finished the set, or completed the game 100%Even if it costs you $$$ to do so i would replay some missions.