The Lava Planet is a area filled to the brim with volcanoes and lava. Its Elemental enemy would be the Lava Elemental. The area would have three levels; Canyon, Lava and Vulcan. Based on screenshots, it appears that this level's gimmick would be avoiding the lava that's everywhere.
The Lava Planet was scrapped when all of the post-Egypt story arc content was removed from The First Encounter. However, a part of it survived into the final game; the Lava Golem. It was originally an enemy for the Lava Planet, but when the Lava Planet was scrapped, it got a new model and became the Lava Golem.
Parts of Planet Kleer seem to be similar to the little info known about the Lava Planet, but whether Planet Kleer was actually inspired by the Lava Planet is unknown.