When some one does a killer stunt, or helps another person a great deal. I beleive a prop system should be implemented, on foot, a high-five or thumbs up maybe. In a car, a wild horn sound, and/or dazzeling flashing of the head and/or rear lights.
Customizable Face, body structure, a character editor, maybe we could be made to look like ourselves, or resemble ourselves. Gives it a more realistic touch.
Car park editor, a map for the players to design to together. Just for fun.
Stunt contest, use the islands, or your own created park, set a time limit for whoever does the most insane stunts (without destroying there cars) in the set time and gain's the most points.
Emoticons... For the newly implemented Direct X chat system! That would be so cool!
A chat system in the dedicated server, make it so that the users are able to temporairaly remove there characters from the game, or that the users have the inability to absorb damage, a brick wall character, (an AFK type thing), from there the option enter a chat in the dedicated server to ask questions, or just to take a break (we don't want to catch some carpal tunnel syndrome, the game is so addicting)
Team mode, if there are 30+ players in the game. Make it so that to freinds are able to team up, a sense of unity...
This all may be to much, or beyond the control of the MTA team, I just thought they would all be great to see happen ...