[IanW]

Interesting idea. Just a couple suggestions:
Unless you are planning to weld this, I'd suggest beefing up the gussets between the siderails. Add a few more rivet/bolt holes to them, especially on the insides.
With cantilevered wheels like this, I'm not sure if having the sprockets on the outside of the wheels would be the best idea. If you can figure out how to move them to the inside of the chassis or even to the other side of the wheels without violating your design goals, I'd recommend it.
Finally, I don't think it should actually be used for competition. If you use it for preseason R&D, the modularity could be beneficial, but for competition, choosing one design and sticking with it is almost certainly better. This way, you don't have to make concessions on other parts of your robot just to accommodate this drivetrain.

All told, very nice job for your first CAD project. Keep taking on new projects - it's an excellent way to get better.