I definitely want to flesh out the Foundationists a bit before moving on to other forces (not counting the Dinos of course).
A light tank and massive mobile assault fortress thing have already been sculpted. I had the MAF sculpted with 3 new turrets whcih will also fit the other tanks, so I will be selling Mark II packs of the old tank hulls with the new turrets also. Here are some renders of the finished MAF and light tank with a Kilgore Heavy tank for scale:
The middle turret is generally for the Kilgore chassis, the rear turret is either for the kilgore or the support chassis, and the front missile turret is for the Trout chassis or the Light tank. The light tank turret could also be swapped out for the Trout chassis.
Other than these I would like to do some infantry, and air support. I was toying with the idea of whether or not to keep them all grav, or if they should have some walkers. I really haven't decided yet.
For opposition forces I originally wanted to do some really sleek alien-looking grav tanks...but the thing is, my digital sculptor does awesome work with mechs. So lately I've been considering an insectoid race with all walker/mech vehicles. The opposition stuff is really down the line, and I don't want support of the 6mm fantasy stuff to suffer.
Oh ... wait ... Here's the thread ... 
Nice that the turrets will be interchangeable with the existing vehicles, too. Also, I've seen Charles' existing mecha/walker designs on his Shapeways account - good stuff!