Well, GTA IV pc is coming out soon! I know everybody is going to mod the hell out of it! So, I have my ideas of what some mods should be.
PACKER: The Packer sucks in GTA IV. Extract the Packer from SA or VC.
Stunt Island: W/E a stunter needs to eat, live, and breath stunt. (Most likely a Total Conversion")
Create-A-Character system: From the MTA they have in GTA IV, you can make a player model, carry that system into the storymode line.
NRG-SA: Everyone knows the NRG sucks in IV, Extract the NRG they have in SA and bring it in IV.
Of course these are ideas. But I only think 1 wil become reality. It is the Packer. More ideas coming up soon!