So I loved this post by Jeff Reints about assembling existing adventures into megadungeons:
how to make a megadungeon without really trying
"Like many DMs, I own a lot of published dungeons. And given the amount of free and cheap D&D material available on the interwebs, we all have access to approximately one gerjillion dungeons of various sizes and complexity. My method now is to plunder that archive and assemble my megadungeon out of bits and pieces of other dungeons."
Yes, this. I totally agree with this idea. Not the least of which is because I discussed doing this a while back (January 2012 to be precise):
"Copy Things, or, The World is Your Geomorph. If you like something on another map, copy it. Either use it as inspiration, or just use it. [. . . ] Need a watery cave? Find a watery cave in an adventure and use that. Need a temple? Just copy the temple. You don't need to honor the original creator's complete vision because you're not here for that. You're speeding up your mapping. You can use the whole thing, of course, but you don't have to."
A couple of places in Felltower are literally just tag-ends that lead to published game material. A couple of other places - includes Sterick's Prison - were just maps and concepts taken, along with much of the existing encounters, taken from published adventures. Sterick's Prison is just level two of S4 The Lost Caverns of Tsojcanth, complete with its go-and-return puzzle. And I started my campaign off with B2.
I also used part of existing adventures for a couple of other encounters. I lifted a few rooms from published materials, re-used rooms from the Dungeon of Death complex I'd populated for my 1st edition GURPS game on the Forgotten Realms, and more. Copy, copy, copy.
This avoids one of the big pitfalls of "Other People's Megadungeons." If you know the source material well - if you've played and re-played T1, S4, C2, S2, A1, etc. over and over, it's not really that complex to mesh them up. You know the material, and you know why you chose them and chose to mash them up. The familiarity is there.
And it's not all that hard, really, to put some connective tissue between them and start running the game.
This is all much easier, too, if you run an AD&D-compatible class-and-levels game. You don't even have to change anything. Just plunk them down and go with them. Modify 10-20% of the encounters and I'll bet your players hardly recognize them.