So, the Dungeon Dimensions are an infinite collection of naturally-occurring hostile pocket universes. Any door in a sinkhole of evil (an area infused with evil magic as the result of corruption) may naturally become a portal to a Dungeon Dimension. Dungeon Dimensions are often thematically associated with the cause of the sinkhole; a sinkhole resulting from human sacrifices to the spider goddess will tend to yield portals to dungeon dimensions containing spiders both mundane and supernatural, as well as other servants of the goddess. Such denizens are spontaneously generated from the matter of the dimension and the souls of those sacrificed to the deity, demon lord, or other power responsible for the dimension. If the sinkhole is cleansed, the portals close, but the dimension remains, and should another portal appear due to a similar cause, it may be reachable again.
Internally, each dungeon dimension is basically what we'd think of as a dungeon level. These might be more to it outside the area reachable from the portal, but the potential infinitude of a dungeonesque can be ignored unless the PCs start bringing pickaxes and passwalls into the dimension. The walls between segments of the dimension might be passable to certain denizens, or there might be a cosmological reason that no reachable segment of a dungeon dimension can be larger than a sheet of graph paper, or dungeon dimensions might be able to contain portals to other segments or dimensions entirely. Such a structure would lend itself very nicely to a node-based megadungeon, and given that the portal network might change over time (as new areas are shadowed and old sinkholes cleansed, or as dark powers change things up out of malice and caprice), the addition of new portals between dungeon dimensions is readily explained.
- A portal to a dungeon dimension has opened in an innocent tavernkeeper's cellars. Clearly evil forces are at work nearby in town... but where? The dungeoncrawl takes on an air of investigation; what are the trappings and symbols present in the dimension? Can they be used to locate the source of the portal?
- An ancient evil is imprisoned in a dungeon dimension, and an evildoer seeks to open a portal to the particular segment where the ancient evil is to be found, in order to consult it, release it upon the world, or what-have-you. Unfortunately, he's not sure of quite the right set of atrocities to get the segment he wants, so he's proceeding experimentally...
- An fabled or important treasure has been lost in a dungeon dimension. If they seek to retrieve it, the PCs will either need to find or create a portal to it. Either approach is likely to entail occult research, to locate a site terrible enough or a group known to create that manner of portals, or in order to learn how to create such a portal oneself.
- A dungeon dimension with portals in known locations might be used as a means of perilous rapid transit. Mines of Moria, extraplanar edition.
- Cultists of one dark deity undertake the rites of another to open a portal to the dimensions of an enemy deity, in order to sack them. This throws the detectives off.
- An evil organization is using a heavily-fortified dungeon dimension as a base of operations. At low levels, the party fights cells trying to open portals in new areas to expand the organization's influence. At mid-levels, they raid bases with active portals, and must deactivate the portal before reinforcements can be summoned or arrive. At high levels, they storm the dimension itself.
- An irritatingly persistent portal in a local graveyard opens of its own accord during the last new moon of each year. The locals know to stay in on those nights, but the PCs might not (or they might go seeking). Perhaps they can close the portal once and for all.
- I like the idea of the mythic underworld dungeon, where monsters spontaneously generate, the doors are malicious, and the geometry is messed up, but I dislike the singularity of it. This approach changes the portal from The Gates of Hell to one of many gates to a tiny slice of an infinitude of hells. Sort of a DCC way to look at it.
- I also like the idea of the interplanar campaign a la Planescape or Magic the Gathering, but I have enough trouble mapping one universe at continent-scale, nevermind multiple. By providing pocket dimensions or segmenting infinite dimensions, I can keep things at comprehensible and preparable scales. Also provides a good stepping-off point to further extraplanar adventure; the Dungeon Dimensions might be easy to reach from the Prime, but they needn't be the only other dimensions in the multiverse.
- My dungeons are entirely too believable. Sometimes I include plumbing for cromsake (sometimes that plumbing is relevant to the game when it is full of green slime). This approach liberates me from the shackles of naturalism while also not really changing much else.
- Allows very nicely for dungeons in urban environments without interfering with the sewer system.
- I like themed dungeons but have a hard time reconciling them with my naturalist sensibilities.
- Reusability. When the door to the dungeon moves and the population reinforces itself even while the dimension is sealed, a segment could be reused in a completely different overworld context.