Utterly Mad

Madness, build a scaled-down Abrams (or maybe a Merkava) complete with adorable tiny main gun.

I was actually thinking about sizing the main gun so it can fire Nerf footballs (the ones with fins on them) as "anti-tank" shells to eliminate enemy vehicles as per field rules, and then just attaching a standard airsoft M16 alongside to act as the coaxial machine-gun. Or, making shells of the same diameter as the Nerf round, put a plastic wad into the new shell, and fill the wad with pellets to make that round work like canister shot.

Yes, I am aiming for a very ambitious project.

Or go halfway and build a WWI tank.

Complete with cast iron German trench armor.

Considering that most WWI tanks generally had the engine sitting in the same compartment as the crew, I don't think I'd want to do that. Plus, the lack of a turreted main gun seems less entertaining IMO.