The AD&D 2E boxed module Night Below is a classic, and for good reason. It offers a huge Underdark area to explore, new and interesting peoples to interact with, including deep gnomes and the undiscovered race of rockseer elves. There was an emphasis more on exploration and role-playing, particularly diplomacy and politics amongst the various factions of the Underdark. It was made very clear to the DM that the PC's choices often would have longterm effects on their relationships in the Underdark.
This box set contains three books, a couple of map/posters, and an abundance of copyable player handouts. It's even got cut-out minis for combat! The three books are "The Evils of Haranshire," "Perils of the Underdark," and "The Sunless Sea." I see no reason that each of these books couldn't comprise a tier of 4th Edition play, albeit probably accelerated. It would certainly require a great deal of work, but a conversion could certainly be done.
And I intend to do it! Yeah, one more D&D project to embark on, and a big one at that. A question for my readers who might be in the know: are the any legal issues that I need to deal with here, as far as TSR's IP is concerned? I have no intention of copying anything word-for-word from the original text, or of selling any of my work, but the plot and such would be essentially unchanged. Is this type of thing kosher or am I getting myself in hot water here?
I know some of the work has been done before by others. The beginning is pretty easy, mostly because the first book is typical, cliche backwoods fantasy adventure stuff. The meat of Night Below happens when you actually get into the Underdark and start exploring. It seems the conversions that are out there tend to be more of the adventure-report style. I need to take some more time to read a little deeper into what people have done, but I'm not sure an actual start-to-finish 4e conversion exists. Prove me wrong!
Perhaps do it as a series of dungeon delves? That would certainly be plenty of work to get my DMing chops up. I'd run into problems at the climactic parts (assault on the sunless citadel, et al) but a series of connected Underdark delves would be a lot of fun...