I'm starting a Mothership campaign and it's a good feeling so I have - understandably - been pondering a Mothership megadungeon. Gradient Descent exists. 

The nature of Mothership got me wondering what sort of megadungeon I could create without the traditional levels and loot, and without returning to the surface with treasure, but still with a delving feel and with ramping challenge.

After explaining both Samus Aran and Simon Belmont to my youngest child in the context of Smash Bros, the idea - trying to stay within the bounds of dungeon23 - is to make a 12 area "metroidvania" dungeon and play around with escalating encounter tables and with re-traversing changing areas.

4 conceptual stages, 3 diary pages each. One a month:

Stage A - players' shuttle crashes into a drifting hulk, power is off. Limited routes available in this area until power is back on (at the end of stage A, to activate the door to stage B)

Stage B - shit gets weird. Things escalate / the situation becomes clearer. At the end of this stage the PCs find a shuttle but there's two things they need to sort before they can escape. In C and D.

Stage C - through stage B. The thing they need to do in C also changes things in A and B (and D)

Stage D - back through the parts of A that were previously locked when the power was off. The thing they need to do in D changes things in A, B, and C.