I posted recently about a Sinking Dungeon and that made me think of the opposite: a tower rising from the ground, little by little, revealing new chambers over time (That's similar to a Floating Dungeon, but not quite the same). Thinking of opposites like that is a good way to generate. Another good way is to force your mind out, to find as many variants of something as you can (like these different types of dungeons). So here I'll try that with the idea of a dungeon that grows.
When I think of growth in this context the first thing that comes to mind is vegetation, crystals, and coral:
A vine or tree dungeon is attractive at first glance. Imagine a huge vine that ancient elves have bred and pruned to grow in a certain fashion, just add water. You carry a cutting into a desert and pour water and green pavilions and lush groves start forming. But, what would be the point of exploring it? If D&D is about finding artifacts (apocalyptic) what is the purpose of exploring the brand new dungeon? I suppose this tree dungeon could have openings to fae realms, but it seem more like a cool magic artifact than a dungeon.
A coral dungeon in an underwater campaign could be the ultimate megadungeon: no need to suss out who built it, no need for a logic of room placement, it's organic and it keeps getting bigger. It will also fill with beasties as it grows, because the lampreys and sea slugs are already there just waiting for new chambers. But who has underwater campaigns?
A crystal dungeon would be cool as an extra-planar locale, and it would grow organically like the coral. But when I think or crystals growing they fill spaces not make hollows. I have a hard time envisioning new chambers being made. Unless it was just platforms floating on a liquid matrix of some sort? That makes me think of lava.
Lava would make for a more vicious Sinking Dungeon, but otherwise I'm thinking of it as being another New Dungeon forming lava tubes, or growing horizontally and chamberless. I suppose if there was a structure the party really wanted to reach, say an obsidian chapel, and it was on a lava field, the lava field's growth could keep pushing it farther away. But that doesn't seem too promising. I've noticed that my first ideas of growth are all rooted in a sense of Gygaxian naturalism (I am a recovering simulationist), but what about other ways the dungeon could grow?
You could have a dungeon that gets larger in one dimension. That sounds like the tower I started with at the top. A sort of Rising Dungeon. While I love towers, they're by nature linear, so I'm always hesitant to focus too much on them as adventure locales. Maybe more interesting would be something like a whole palace rising from mud or sand. It could come pre-filled with nasty mud skippers and slightly damp treasures.
That still seems pretty naturalistic, what if the dungeon grows by two dimensions at the same time (I think three would be hard to do without a computer)? So imagine hallways and chambers get bigger over time. It is an Expanding Dungeon. Bigger rooms don't seem very interesting, but if you designed cleverly, I imagine you could have new parts of the dungeon open up as tiny cracks become corridors, as a thin stone arch fills out to a sturdy bridge across a chasm. Of course small rifts in the floor could become chasms, too, creating new obstacles.
I suppose another idea of dimensional growth would be adding dimensions, a mural or blueprint that takes on a third dimension. That seems more like a magical artifact too, though.
There's also the dimension of time. I love the stories of people disappearing into the fae realms and time working differently for them. What if our dungeon has advanced time passage and players know this going in? "Look, the sage said every hour in here will be like a month passing outside, so let's find the brass urns and get out!" The opposite could be cool too, maybe they know a dungeon where time doesn't pass and they use it to hide out from powerful foes, but they have to deal with surviving the dangerous denizens of the dungeon meanwhile. Or maybe the party-as-a-whole decides to fast forward your campaign time-line, that could get interesting.
This is related if you think of dimensional in terms of astral/ethereal planes. What if a dungeon emerges slowly over time in solidity? A party might need to hurriedly explore the misty walls in order to position themselves well for the final solidity. This assumes they aren't just trying to escape, because then they could just run for an exit, right? So why would they need to position themselves? I don't know, maybe a temple or artifact will be emerging too and is unreachable when all walls are solid. This dungeon be "tidal," cycling though solid and shadowy forms.
That makes me think of ice melting and freezing. It might be a different version of the sinking dungeon, the ice dungeon loses upper levels over time, dropping treasures and monsters on top of each other on lower levels. But, wait, this started as me exploring growth. So, a growing ice dungeon seems like it would be similar to the crystal dungeon, or it could be similar to a lava field. Umm, I'm drawing a blank here.
Okay, I'm running out of steam and I realized I didn't talk about a meat dungeon, like being inside a beast. Any additional dungeons that would grow?