If we have people willing to do it, I'd suggest 3 different vehicle types based on the themes of weapons featured in Chaos.
For the medieval weapons, you could have a medieval vehicle of some sort, perhaps some kind of catapult or war engine? Then you could have an ATV or a buggy or something like that, and then maybe some kind of futuristic flying hoverplane or something?
oh you mean a ballista. there are some ballistas (Giant crossbows) in the beginning of the gladiator. you are talking about a giant one that has arrows that can nail 2 people at one time? the ballista would have to be hella big...
Make the arrows explosive (like the crossbow's exploding arrows), and you've got a nice little piece of artillery, capable of taking out more than just a few players. Course it could take some time to reload, but it could be the most potentially damaging vehicle-based weapon in the game. Then, the more modern vehicles could trade weapon damage for armor.
A spider walker. 4 legs and a turret in the middle like those turrets in the ut assault except it can be a minigun or something. or a ufo that shoots "probes" that seek out people.