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 Goliath (a tank) and Metal Gear Gustav (a smaller non-nuclear armed model of Metal Gear). There are actually various Goliath tanks inside Zanzibar Building in the final version, but they're just part of the scenery. The concept for Metal Gear Gustav was later recycled into the Gekkos from Metal Gear Solid 4: Guns of the Patriots.