This is way out there, but a new vehicle type being bipedal robots you can drive around in.
This would require the following:
- Walking animations and motionless turning animation
- Particle effects coming from the Feet node (when it stomps on the ground, poofs of dust and possibly small tremors occur)
- bobbing effect for the rider
That's pretty much it actually, dealing with weaponry would come afterwards. It would just be a very heavily armored, slow walking mech.