The Watering Hole - A much-needed pit stop in an extremely inhospitable part of the world. Like, just before the DF2013SW equivalent of Mordor. Think "The Nexus" from Demon's Souls.
Spiderville - The baron of this once great mountain hall liked giant cave spiders... perhaps a little too much; he adopted an army of wild GCS and gave them all names. Long story short, these spiders are all that live in the fortress now.
(you can substitute any other cave creature for the GCS, but I guarantee it won't be as !!Fun!! for the adventurer!)
Tower of Glass - A castle/maze with hostile inhabitants where all the walls and floors are see-through. Bars, windows, fortifications, and supports make up as much of the construction as physically possible. Preferably above-ground so you can actually... you know... see stuff.
The Final Boss - I'm sure other people have thought of this, it's an obligatory part of the Mordor experience after all. Immortal vampire/demon-necromancer adventurer legendary in all combat skills, equipped with a pair of artifact weapons, a zombie/goblin/mercenary army, and his own private fortress. Wanted by every civilization, having been responsible for a whole lot of genocide.
End of the World - A cryptic structure located at the very tip of the most distant corner of the arctic world. One can only imagine what horrors await within.
(it can be so foggy in the arctic that you'll only be able to see 1 tile ahead of you before going underground, and your companions have a habit of "disappearing" along the way, it's spooky as hell
)