I think it would be cool to have a TinyDuino robot chassis. Just the motors and a good way to mount the TinyDuino. I've seen a few inexpensive miniature 3v motors. These would go well with a rechargeable  battery and the Tiny Motor Shield. I haven't seen any TinyDuino robots so far and maybe thats because there are no chassis to start with.

Any particular style you have in mind?

I don't really have any particular style in mind except that it needs to be easy to add to. It would have to have many places to add new sensors and other components.

This page has a bunch of nice examples:
http://www.conrad-electronic.co.uk/ce/en/overview/2420180/Build-Your-Own-Robot-Kit

We have one like this:
http://www.robotgear.com.au/Product.aspx/Details/451-Pololu-Robot-Chassis-RRC01A-Transparent-Gray

it works pretty good