1. Add vertical rotation so you have car movement more synced: If you are passenger on a PCJ, and make a turn you notice you flip out the corner, as driver you flip in the corner.
2. No bullet tails (With cooperation with suggestion #3)
3. Let other players control your health: If you hit a player, you send the health of the player who you shot. This means: if you shot a player on YOUR screen, you actually shot him and that does maybe mean for the other player you missed on HIS screen. If you remove the bullet tails you can't see he missed. This avoid's 'lag' and you dont have to shoot forward. I beleave this is the way they do it in CS.
With the last suggestion can be dangerous for hackers crackers and other scum, but i think it's great.