I have been working with omni drive systems for years. I first come up with the design in 2002 but in 2004 we built one and wone a Xerox creativity award at Great Lakes. Visit our webb site for pictures and tech tips @
www.team1322.org . Per your Idea about tank track omni drive, your idea is a good one and thier are different ways to make it work but the key is when you install the two tracks for your drive you can add two more at 90 degrees. Doing this it will look like a four sided drive. When you drive the different motors it will allow you to go side ways forward, back, angles and rotate. Thus you have a omni directional system. With the number of wheels you will have good traction and can become a pusher robot. the only problem is keepng down the weight. We used omni wheel drives with the motors in the wheels.
http://team1322.org/robotics_04-05.htm