I would like to see some kind of attack/defense modes. These could "simply" be open maps where one team has to race to a finish line, while the other team tries to stop them.
With weapons and boosts the defenders can shoot and ram hostiles and cause them to teleport back to their spawnpoint, the attackers have to come up with a way to pass the defenders by, leech them from their weapons, shoot them to mess up their aim or simply drive by them at high speeds.
This can also be a CTF mode, where both teams need to capture a flag. Hit an enemy? He drops the flag and you can pick it up. Ram an enemy? With enough force (not a lot) the one carrying the flag will drop it. This causes small fights where both racers keep ramming each other to constantly drop the flag, then hope they have the acceleration and direction to get away with the flag.
The flag could be a powerup that can be fired, so you could potentially fire it to an ally who catches it, or it could be intercepted by an enemy.
Both versions could add destruction, where each vehicle has a maximum amount of health before blowing up and respawning. This allows a team to stop an attack, and adds more depth to player choice of using an armoured mastodon or a zippy little thing made out of wet tissue paper.
Both versions could have maps filled with pickups, boosts, jumps, various unfinished tunnel systems (having to drive on the walls/ceiling while avoiding holes or missing strips of tunnel, possibly only reachable with jump ramps) and environmental hazards. It would require a fair bit of players to work I think, since it might be too easy in small populations to stop someone racing away with your flag, or even just keeping them away from your flag altogether.