Over on the blog Monsters & Manuals there there is this post about The Mystery of Ideas. Basically, how you can't force them to come to you.
I get this - you have to cook while the pan is hot. You can't really force things to come, although you can learn to set the stage to take advantage when they do. It can be frustrating when you need something and can't think of a good idea for it.
This is actually one of the reasons I really like having a megadungeon.
Since the place is big enough for almost anything, anything I do think of I can stick in there . . . somewhere. No idea is really wasted.
Not only that, those areas mapped out and stocked up from those ideas stay there, waiting for discovery.
Still further, the work is cumulative and ongoing. I don't need to be done. I can do the pieces I have ideas for when I have those ideas. When I don't, I can move on to the ones I do have ideas for.
You can put down placeholders as well, something that'll do until you think of something better. Sometimes you do, and you can improve it, or suddenly realize why the placeholder is so appropriate for the spot it's in. Sometimes you never do, and the PCs roll into and over the placeholder. Even then it's done its job. Your players might come up with an explanation for it that's better than anything you would have.
It's just one of the little things about a big, game-central dungeon that I like. No idea is really wasted, and even lack of ideas isn't really a big deal. It all adds up, cumulatively, to a good experience. Felltower has gone on for 50 delves into its depths, still draws the players to the game, and still has mysteries to yield up. And it still provides a place for all of my ideas. That's something I didn't know I was missing, growing up in a module-and-distinct-adventure era of gaming.