It seems like this would be best solved by creating an arena where legs had an advantage? The whole advantage of legs is that you can traverse areas that aren't unnaturally flat (almost all areas in real life).

Make every single arena a completely flat space where wheels are optimal and then given extra points to sub-optimal builds seems like it would be a poor way to approach things. It would be like giving extra points for creating an aquatic robot, but failing to create an arena with any water in it.