I've been roughing out an idea that might fit well with this. Due to lack of time I haven't got very far with it (I know, a common excuse, but I really have been extremely busy, and it's not going to get any better in the immediate future), but I'll throw it out there anyway.
I was thinking of an area that used to be an old "Citadel of the Mages". Long since abandoned, it was sealed up for the safety of locals and largely forgotten. I have a background story roughly worked out for why, but I won't get into that now. I envisioned this as a three dimensional maze of corridors and rooms. The reason for choosing a 3D maze is that it's different, and it could be made reasonably hard (as hard as the green maze, for example) with relatively few rooms. The maze would have paths that led to a common area, with kitchens, library, etc, and paths that led to the quarters of the long departed mages (some of whom are perhaps not long departed after all

). Some of the rooms would be above ground, and some below ground. Very few of the rooms (and perhaps gardens) that are above ground would be connected above ground though. I was going to design the layout of the maze in CAD software, because it would be easy to envision the interlocking structure of the maze, and then build the rooms from that layout. The general theme of the rooms would be "decayed opulence". The paths that led to individual mages quarters would each have a slightly different theme, as a clue to navigation. In some places these paths could come to the same room, but bars in the rooms would only allow you to see the other path, not get to it. There would be a lot of clutter and junk left around from when the place was abandoned, which you find just by stumbling upon it. Most of this would be useless junk, but there would be a few nice to have items too, which could only be found by careful searching or a lot of luck. The monsters would all be versions of common monsters that you might expect to find in such a place (spiders, beetles, snakes, etc), but mutated by the remnants of the mages experiments, chemicals, and equipment into something a whole lot worse than their distant ancestors! The house could be the entrance to the citadel. There's plenty of unused map space for the citadel itself just west of the house, above the cliffs.
As far as the idea goes, I have more details than that very brief outline, but still no complete picture. One thing that concerns me a bit is whether this really fits with AT, which has no magic as such. If that's a problem I think it could be adjusted to fit though. Alchemists and potion makers instead of mages, for example. I have created some of the items that would be in the citadel, but that is the only content I have actually created. The reason I'm throwing this out is because I know I cannot really take this to usable content in any reasonable time frame without a huge amount of help (by which I mean, I will be doing some smallish percentage of the work). Sorry, but that's just the way it is right now; I'm in the throws of starting a company, and that consumes a lot of time.