I have a few ideas floating around in the ether of my brain. As soon as I can really get them distilled into something concrete, I'll post my ideas. Just a couple quick ideas so far:
1. GreyOwl mentioned only unique heroes as pilots. I don't think that is really necessary. I think I understand the why of it, but there is a part of me that sees a squad of police standing around until their police seargent hero comes to drive the car.
2. whitestuff mentioned size. I think that it should be mediums can drive. IIRC that eliminates pretty much all of the non-humanoid figures. We'll just say that dwarves can't reach the pedals
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3. I think that each vehicle could maybe have a climb number. It is a number that represents the highest number of levels a vehicle may climb per hex moved. The average car might have a climb 1. A 4x4 might have a climb 2. That might be something that is a safe descent number as well. An unsafe descent risks a crash or runaway car situation.
4. I think that the driver should get the order marker and the car is viewed as an equipment card. When the figure is in the car they use the car stats instead of their own. I think there ought to be some sort of bonding idea where a passenger may use a ranged attack from the passenger seat, maybe with a -1 attack penalty. Possibly a fire anywhere during the move as long as they have LOS. The guy manning the machine gun turret uses the vehicle's special attack instead of his own.
5. I agree with whitestuff that vehicles should be allowed to end on uneven terrain. If there is a climb number used then that is the limit on uneven terrain. Something I have been toying with is to just get some sort of singel hex base to attach to the car and just treat it as a single hex object. It's something that might simplify the whole idea.
6. whitestuff mentions getting into the car. I would say that the car should be treated as a space that can only be entered from the door side. Once in that space they may at any time open/close the door. Maybe throw in a condition of not being able to open/close the door if they are engaged. Once in I would say they should be able to attack normally. Maybe if the door is open the attack/defense numbers are unchanged. If the door is closed the attack number drops but the defense number goes up.
EDIT: I really like the turn idea, GreyOwl. My only quibble with it is that a zig-zag line is necessary to approximate 4 directions (n,e,s,w). The turn number kind of kills that idea. I think in the end just sticking with the 6 natural straight lines is probably the way to go, but the zig-zag line to get the perpendicular might be something to consider.