I did a search for this and found some stuff somewhat related, but not completely what I was thinking.
I'm not sure if any of you have ever played the game Vigilante 8, but I think it'd be pretty sweet to have a mod like that.
Basically a Vehicle DM. Start off with a car, bus, or whatnot, drive around with a flame thrower, rockets, or something other than the default smg weapon while in a vehicle. Instead of aiming out the vehicles sides though it may be possible to fire from the front of the vehicle (like a bike).
No idea if this would be relatively possible or not, but yeah, I think it'd be cool. Thx!
-RaFF