I think that rising world should have a slight fantasy feel, and i think that it would be really interesting to have some kind of strange forest deep underground. There could even be a few types.
1- Coral-like trees which give off an eerie glow, and can be harvested for strange flowers. This could be fairly deep underground, but common enough that most miners will come across this at some point in their travels. I imagine this biome as having a really peaceful feel to it. the trees would groan lazily as they swayed, and the glow from the trees would make it feel like being underwater.
2- Giant Slimy toadstools and moss-like coverings on the stone. The slime can be used in potions and perhaps to create waterproof coverings for clothing. The moss could be an ingredient in cooking perhaps among other uses. This biome could be a lot rarer that '1'.
3- Deeper still, and to indicate that the hell biome is quite close by, this biome could have a variety of poisonous-looking plants and trees. It could even have pools of black water, which can be used as a flammable coating for torches. This biome could be a little unstable, and rock 'teeth' from the cavern ceiling could rumble and come loose to add an element of danger. Hell would be quite a lot further than this biome, but the environment could begin to change more and more like it, until you arrive and go,"holy crap!"
I think that biomes like these would break up the visuals of the underground layers, and cause players to really want to get down there. haa!