ToME WikiMushroom circle - Similar to fountains, when you "drink" from this new terrain type, you instead get a specific type of mushroom. Restoring mushroom patches would be a very handy thing to find mid-game. This could be restricted to a forest-type terrain, to prevent them being generated in Mount Doom... 
MassimilianoMarangio: I like the idea, but searching the terrain should produce the mushrooms and not "quaffing".
RavenRed: Exactly! It's an adaption of the existing fountain code, and provides interesting... uh... flavour. A Mushroom Circle of Athelas should also be possible, I'm thinking (although it's not exactly a mushroom). Or were you thinking more of what they have with finding items under rubble? (i.e. if you're burrowing through trees, you have a chance of uncovering a Mushroom Circle?)
MayLith: Completely different concept: Make it a *true* Mushroom circle. A circle (or box) of Mushrooms... either all identical, or maybe all different. The radius of the circle depends on how old it is -- gets bigger over time. If the circle gets too large, it all goes away again.
RavenRed: Perhaps a "King" or "Queen" mushroom, with the "Escort" flag?