Subdungeons are awesome. They're probably my favorite new addition to the beta. I'm surprised there hasn't ever been a thread with subdungeon edits and new ideas! So I'm starting one. Here's some of my own thoughts:
General: Make the exploding signpost do more than one trick. Perhaps poisoning them, turning an item into gold, fill an item slot with a poltergeist item that does nothing.
North: Make a subdungeon with an imp in the middle. When you talk to him explains he's and alchemist and ask for 25 gold. If you give it to him he will do one of two things: 1) Take it and disappear laughing. 2) Return the favor by giving you two random alchemist scrolls.
East: You enter into a subdungeon with a gold stash on the other side of a green indestructible wall, with a level 10 AA guarding the only crossing point.
South: A subdungeon with a bunch of random staircases, like tiki tooki's subdungeon. Standing on the entrance to each of those tunnels is a level 9 imp with 100% resistances, that does 50 damage. The imp only blinks between tunnel entrances. In one of the sections is a WEYTWUT glyph.
West: A subdungeon that is divided into two 1x2 areas, with a lava wall in the middle. On one side a level a 9 wizard blocks the Berserker Cudgel. It give might against magic enemies. On the other side a wall blocks rock armor. It makes stone skin give magic resistance.