I like the OP. It doesn't even need to be fantasy, just Rising World original. I would love more unique biomes underground. I mean yeah it would have a fantasy feel but only because it would be something not found on earth. It could still have a realistic look so it fits with the RW world. Make sense?
Heaven biome? Well no, not something religious (I've even suggested Hell should not be called Hell..). But I like the idea of some biome being up high somehow. So I thought of a biome being high cliffs which cannot be climbed and on top is a plateau. While on the plateau, if you looked off the edge all you see is cloud. This biome would be another RW unique one with flora and animals which cannot be found on earth. To get up there you must find caves which lead up or just tunnel your way up there yourself.
Issue with this is the game's world height, not sure if it could be increased enough to support this idea.