Obviously i would like to see a smooth running multiplayer to begin with, something that everyone would be happy about. Cutting down the lag to the lowest effect of gaming over a online game would put a smile on quite alot of peoples faces.
AI activity to be involved also. Doing missions online, being apart of a gang, making a name for yourself, fighting of the good of justice or being with them giving you the player a whole range of missions which needs to be accomplished to make yourself the #1 person.
Gangwars would be a very interesting feature also, including with the AI activities just getting on with their normal lifes. You could shield yourself by hiding behind the pedestrains, distracting the opponents etc. or if your the good of justice your mission would be to wipe out the opponents by calling the forces to help you out, maybe including AI helping also.
Hours of fun perhaps or not heh ;P