I'm not sure about vehicles, but I think in CTF or A&D, a few stationary weapons would add some fun. Like a cannon that's bolted to the floor in a base, or maybe just one in mid-map. It would have no ammo until you brought some for it. Maybe the ammo would spawn in the bases, but you could only use the ammo that spawned in your enemies base. Or maybe it would load itself at random intervals, so it might come on line when you are being pressured in defense, or it might not.
It would be a pretty big departure, but I think it could work in certain team-games as an optional inclusion. Vehicles might also add some fun on these game types. Not something with weapons or something that totally protects those inside. More like just something one guy can use to bring a bunch of his team mates to the enemy base. It would spawn mid-map at random intervals or maybe spawn on a weapon spawn but randomly and very rarely. I think it would encourage team play on public servers rather than just have a bunch of FFA-ers all trying to be the one who gets the flag and doing their own thing.
Speaking of which...a one time only "team talk" option might be good on all team games. I know you can have a timeout, but calling it that, just leads people to think you've gone to the toilet. Anything that encourages teams to work as an organised team, has gotta be a good thing