I'm using this opportunity to update my first megadungeon to modern rules via an entire rebuild. I'm not going to hold myself to a room every day (because I have a life), but I will hold myself to 28 to 31 per month. Before it begins (tomorrow) I am respecking the plot, major players, and storylines. I will revise as I go, but I already know that:
1.) The 'dungeon' is an island with a village, fort, mansion and cavern complex under it - and mirror versions in the Shadowfell (a Ravenloft domain), Feywild (the abode of a Trickster Archfey), and the Ethereal Plane. It will go into the Underdark, as well as connect to additional planes. (Elemental, Hells, Astral).
2.) There are villains aplenty. There are short term low level groups with a bad guy leader, medium level powers that will stand in the way of the PCs, and high level threats that lurk in the shadows. I know what each wants, what each knows, and where they'll be located, roughly.
3.) Because it is 4 overlapping mirrored planes I will only need about 120 layouts - but many of them will be used 2, 3 or 4 times with modifications.