One of Kojima's early plot ideas for Metal Gear 2 involved a terrorist takeover of a nuclear weapons testing site within the United States. This idea would later recycled for Metal Gear Solid.
Two other vehicular bosses were planned in addition to the Hind D and Metal Gear D. The Goriate (a tank) and Metal Gear Gustav (a smaller non-nuclear armed model of Metal Gear). There are actually various Goriate tanks inside Zanzibar Building in the final version, but they're just scenery. The concept for Metal Gear Gs was later recycled into the Walker Gears in Metal Gear Solid V: The Phantom Pain.