The first one I'd like to see most the most, but I think the others are decent as well.
1) A cargo aircraft that can carry several teammates or if only carrying the pilot, can pick up a goliath, leviathan, or hellbender. Dropping vehicles from excessive heights or at excessive speeds would have to result in damage or destruction to the vehicle, if it doesn't natively do so. Perhaps even have it carry a full load of teammates in its hull as well as being able to carry a vehicle. Carried vehicles could (hopefully) be manned, or perhaps unmanned to spare coding headaches. Certain or all vehicle weapons would be disabled during flight. An upscaled and widened human space fighter could probably serve as the mesh if need be. In order to maintain balance it wouldn't have any substatial weapons, if any weapons at all. It could fly like the manta (though slower and less maneuverable) but higher above the ground, perhaps, crouch button to lower to grab vehicles or for crew loading (this would be dangerous at high speed as rubbing the ground would be less forginving damage wise to the cargo craft and/or attached vehicle than the manta). Jump button would be for a temporary burst to clear steap slopes, left click would be to attach a vehicle, right click would be to unattach the vehicle.
For the sake of balance, it could not be repaired, to further limit it's use, and make for a less often and more unexpected surprise attack. It would not spawn very often at all, and should probably only be placed to spawn from a base. It should also have a massive explosion that would damage a vehicle if nearby, and destroy any vehicle if it were still attached.
2) a modified (additional, not substitution) manta that would carry a goliath like cannon. (though somewhat weaker). As the cannon would be heavy, the manta would not be as maneuverable (mostly it wouldn't rotate as fast). Also, the recoil would violently hurl the manta backwards. As a safety mechanism for the vehicle, the cannon could not be fired while the vehicle had any backwards velocity. This combined with a slower acceleration and turn radius would naturally greatly limit the vehicle's rate of fire to much less than that of the golaith.
Ground rubbing damage should be amplified to have negative consequences for shot from a badly chosen location. Vehicle hit points may need to be lowered for balance if the slow rate of fire isn't enough. Increased ground damage and lessened manuverabilty should make it so road rage kills are much less frequent (as it would be easier to get out of the way) and further balance the vehicles overall offensive capabilties.
3) Motorcycle: primary use: to get around FAST (fastest vehicle). Weapon: none. Driver must brake to within a certain speed before dismount. Ramming into an object at hight speeds will detroy the bike and the driver (thus no killing sprees because no one has low enough ping to take out the high speed bike). Hit points would be low, but not too fragile.
Primary fire: Nitrus boost (very slow rate of fire, very fast boost)
Secondary fire: Sharp turn (only effective at slow speeds)
Without secondary fire pressed, turns are very slight due to the high speeds.
4) The MHARS, (pronounced mars) or Mobile Healing and Resupply Scorpion. It fires weapons with small amounts of ammo. Will replicate and fire a short distance whatever weapon _pickup_ (not projectile) (certain weapons cannot be replicated though of course, like redeemer, etc) was equipped by player during vehicle entry. Contact of weaponpickup by any player will of course add to that players inventory.
Friendly-fire-link gunning the vehicle will NOT heal the vehicle, but rather the person linking to it. This will actually damage... er... deprive the vehicles energy core as will replication of each weapon fired. Left clicking launches weapon-pickup, right clicking honks horn (should sound different than other horns) Slow rate of fire. As the vehicle cannot be healed and is damaged each time it produces a weapon or heals someone it is of course, limited in its resupply/healing capacity and cannot be used indefinately. Exausting the vehicles energy will cause it to explode, so the driver should be carefull to bail out when it is low on energy, especially if someone is healing themselves.
Does not come equipped with blades as it is designed to be driven next to teamates. Nor does it come with an offensive weapon. Enemy link-gunning the vehicle would damage it as normal. Hijacking the vehicle will make it heal the team of the driver. Cannot heal unless a driver is in the car (to prevent a driver from selfishly taking it for his solitary healing use).If you like any of these ideas, please let it be known as sort of a vote for the Choas team to take note of. If you don't like them please state why and suggest what could be done to better acheive balance. And please post any of your vehicle own ideas.
. I think it'd be cool to have a speedboost for primary fire and mabey like a jump or wheelie for alt. fire. Not to mention Big honkin' blades all over it that you could drag the dead bodies of weaklings around on 
. It would be faster than the hoverboard but couldn't go over water and would do more damage to the driver if he crashed.
hmmm... dual miniguns on a hoverbike....
(or firstperson -whatever).