The third level of the ASE will be water-themed. That river in the cavern in the second level flows down into a cataract, which spills into a 500' by 500' lake. The lake has, in turn, overflowed into large portions of the dungeon level.
This level is going to have a lot of those 30' wide corridor with the round ends. I like the look of them, so I'm dumping in a bunch of them. It should feel very, very, very big when wandering around. Lots of gigantic empty spaces. It makes my players panicky, and that's good entertainment. For me, at least. Side corridors off these big halls will lead to more cramped little dungeon rooms full of treasure and death.
There will also be one of the big, lit-up entrances to the outside world on the third level. What's interesting about the entrances is that the higher up the mountain they are, the less deep the dungeon is. So the easiest entrances to get to for the Unyielding Fist, various wizards, and other adventuresome types are into the most deadly levels.
Up this high on the mountain, the Moks will have made a foray. Their presence will be fairly feeble and battered, as watery dungeon monsters find Moks to be absolutely delicious.
Not sure what other creatures will live in the third level. Maybe some sort of gill-men, a la Creature from the Black Lagoon? Possibly too overdone.
I'll have a small Hive Mind and his goblin servants for sure. It's time to introduce those brains-in-jars.
I think I'll stick a pyramid in there somewhere, too. Who doesn't like pyramids? If you're the guy who doesn't, I don't want to hear from you, so don't answer.