Well, its a bit late but I just had this random, interesting idea for drivetrain building; no idea if it's legal though. My team just ran a kiwi omnidrive, which worked beautifully IMHO with correct load balancing (Our last 3 games, we had no practice with the properly setup drivetrain). Meh, I know we could have bridged too with that, just had no practice.
Anyways, what I was thinking was adding a fourth wheel (or duo), traction type, in the center of the bot. It would be raised/lowered by a pneumatic piston connected to the front bumper (ideally jutting out a bit from the main frame structure, via frame perimeter extension sitting on top). The idea is that omni drives inherently can't push as hard as tank; this would enable tank drive on that platform with two powered rear wheels by just putting them down with the piston to a set distance via reed switch to engage them. Specifically with pushing, you could hyperextend the piston when holding ground. What I believe theoretically happens is they apply pressure to the angled piston when pushing it, which just forces the traction wheels more into the ground to make their push harder. Only drawback is you might have the bumper jacked up a bit at times, and I dunno, is that legal (assyme the bumper height is not exceeding the bumper zone)? Technically you do still have a wheel in contact with the ground, not just a stick.
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