Crevices generated in the black walls of caves could be another set of potentially more challenging dungeons.
Beyond level 3 they'd have a chance to spawn, and only spawn in natural cave regions, obviously scaling the deeper they went down and potentially having slightly different architecture/contents/uniques from each level.
But basically, the dungeon, instead of beavers, would be Trolls and their spawn, very tiny trollets, the final boss being a Troll King and Queen (or what ever you want to call them). The enemies could even be randomly genenerated, some being big, slow, powerful sapient mushrooms (think the big fucks from dark souls) and beyond. If you wanted to get creative, lorb, even boreworm hives.
Each would, like beaverwood, have its own architecture, lamps, various doodads, in the case of trolls and mushrooms maybe even furnishings and whatnot.