i just love the idea of porting a top down gta (my favourite would be gta 2 though) to IV or san andreas
if i remeber correctly bobesz converted the first gta2 world to san andreas, but there were lots of collison problems and his project never caught much attention, and after some time he stopped it. sadly
as for the camera: top down is a must but also the third person cam would be intersting Imo because you see how the city looks like in a different perspective no matter how creepy the textures are.
i really hope that someone decides to start working on this i would love to help but i have no modding experience at all...