[lukevanoort]

If adding omnis isn't possible, just reduce the distance between the drive wheels so the wheelbase is significantly wider than it is long. Some casters may be necessary to add to the robot for balance. Actually, if you had casters on the front and rear with drive wheels in the middle, you might (depending on drive wheel squishiness) be able to raise/lower the casters a few mm to reduce the normal force on your drive wheels. Another option might be switching to lower traction tread on a pair of wheels. McMaster-Carr has a wide variety of conveyor belt materials, I'm sure you can find a lower traction one that can be adapted to your wheels (like the PVC or Nitrile Incline conveyor belting). In my opinion, your best bet is to move the front wheel back to a couple of holes after where the idler currently is, and then move the other wheel a few more holes back to get a centered drive system with a width of 1.5*length. Casters at the ends would restore balance.