I've been working on a Rover 5 platform that I go from sparkfun.com. There are a few different variations, but the '5' has 4 motors, each with it's own quadrature encoder, and is tracked. Since it does have independent 4-wheel drive, you could use mecanum wheels for an extra level of maneuverability.
I've been stuck in the same situation. The robotics hobby has a absolutely huge lack of tracked chassis to choose from right now. You have your selection which don't meet any of my needs really. So it boils down to buying old/new RC tanks and ripping the guts out. The Tyco Fast Traxx line look like a good tracked platform if you can find any. The RAD 2 robot looks pretty good as well. However, even in the RC toy line, not many people make tracked anything anymore. This has been the most frustrating thing about making a tracked robot for me. The chassis is impossible to find unless you want to spend upwards of $300 for something that goes above and beyond a basement hobby.
You have your selection which don't meet any of my needs really.