So I've begun work on my Megadungeon (currently unnamed) and have a few more thoughts on Minaria.
Instead of trying trying to get a full concept nailed down before starting this morning, I decided to jump right into the mapping. And boy am I glad. I only have about 30 rooms and hardly any of them are keyed, but I've been thinking in chunks rather than specifics, for now.
The main adversity is currently a clan of mole men (reskinned kobolds) who maintain and repair the dungeon. They are the once who reset traps, are commissioned to build new constrictions, make sure the dungeon doesn't fall into general disarray, etc.
There is also an abandoned shrine to the Carcosian Snake-Men, the original creators of the dungeon's lower (although not lowest) levels. For whatever reason, the cultist-archeologists have relocated to elsewhere within the complex. What remains of their lair will be inhabited by some other type of sentient creature (yet to be decided), as it is a very defensible choke point. Some options I'm considering are a rival cult, a tribe of Lizard Men who believe themselves to be the last living ancestors of the Snake-Men, or a clan in insane dwarves driven mad by an artifice of the lower levels.
The most terrifying monster I've placed to far is a white ape (as per Moldvay, but with 4 attacks instead of two--cause my white apes will always have four arms) that guards the passage down to the fourth level, which is very easy to access through the central shaft. I wanted a main passage down towards the middle levels of the dungeon that could be quickly accessed, but also some warning (the white ape) of what waits beyond. That way higher level characters aren't forced to keep trekking through the same sections of the dungeon in every expedition.
As far an Minaria is concerned, I've decided that the small numbers are the bottom of the major cities/castles is an indication of relative influence and military strength. Also, I'll likely be remaining Elfland to something a bit more pulpy, such as Erithol or Artaina.