The game offers a cover feature already, but for peds only.
Would be really nice if someone finally implement a cover system similar to Manhunt's one in GTA SA! I saw lots of attempts, but all of them gone.In my opinion it would be nice addition to GTA Underground.
Oh yeah, I've noticed that howeverit's not available for CJ to use,that's why I have said would be really great to see this feature in future GTA UG, 'couse this will push it closer to HD era games. I should mention fighting system in bully as well! BTW it seems not to be really hard to create as a script, I have tried, unfortunately my hands are both left... However, I believe it's not so complicated for your crew members, isn't it ? Additional feature as a cover system will definitely change MP way of playing increasing diversity of moves etc.