After way too long trying to get my FLGS to respond to my inquiries -- they mostly care about board games and their RPG slots are largely gobbled up by a guy who made his own game and wants to spend all of his time running it -- it sounds like this will finally be happening this spring.
It's possible I'll rotate through multiple games, likely Shadowdark, Pirate Borg and 5E. My plans for Pirate Borg and 5E initially will be to run published material like Buried in the Bahamas and One-Shot Wonders, which all can be done within the three-hour game window pretty reliably.
For Shadowdark, though, after I run Tomb of the Serpent Kings, I'm thinking of building a mega-dungeon made up of sublevels big enough to complete in three hours, including returning to town and buying equipment and carousing. (In other words, no one gets the light spell and the group gets two torches each adventure, so don't dilly dally too long.)
A three-hour section of a dungeon, in my experience, is actually a pretty small number of rooms, unless most of them are empty -- basically a five-room dungeon plus maybe a few empty rooms to flesh out the space. That's pretty small, so I might go with putting two sets of five-room dungeons on the same level, letting the players have some choice of what to do and what to explore each week.
The resulting mega-dungeon will probably be a little strange-looking, since it's purpose-built, with a lobby area that makes it easy for a group to get to this week's sub-level quickly and easily, although I'd want these sub-levels to also have at least one way to go up a level and one way to go down a level, just to let players get in over their heads or to find alternative ways to escape if something happens to the entry route they used on the way in.
Still noodling on this. I have had some ideas for mega-dungeons in my head for a while, but I'm not sure if this structure works with all of them. I might go with a classic Castle Greyhawk vibe of classic D&Disms or something more Diablo-themed, which I think would also work well.