If it was a tank, you could have Scouts being able to just dance around it with their speed, Sollies and Demos would obviously blow it up, Spies could sap it, Engies could repair it, Pyros could reflect the shells and Heavies could just spam it with their miniguns. The only classes which wouldn't really have any way of working either with or against tanks are Snipers and Medics, although if you made teh tanks open-topped, the Snipers could pick off the driver.
But yeah, no maps are big enough to cope with that really. The classes themselves could probably work with tanks, but the rest of the game not so much.