[Chris is me]

I really admire the minimalist simplicity of this design. Exactly everything a drive base needs and nothing more - nothing overly flashy.

I think your drop is a little severe especially for a 31x27 chassis. The .125" drop-center standard was crafted in the 38x28 era, and on my last robot of similar dimensions with Colson wheels I thought it was just a bit too much rock. 1/16" or even zero drop would probably work well. I don't think it's really going to hurt your performance one way or another though.

Do you have a system to tension the chain when / if it stretches?

The integrated gearbox is great for reducing part count but not so great for serviceability and relies on your sheet metal shop's hole tolerances being as good as your milled plate tolerances. You also have less ability to redesign if something goes wrong. I would switch to a 2-plate gearbox just out of paranoia more than anything else.

Any plans for a belly pan? You could really use one for rigidity? You have very little cross bracing.