Dungeon crawling is fun. Killing the dragon at the end of the dungeon is also fine, but after the player characters have looted its hoard and left, there's a fine piece of empty real estate that shouldn't go to waste. And if the players don't take precautions to secure the dungeon, someone will get just that idea. But who?
Well, let's roll for it. Makes for more surprising results than just choosing. There are four steps to check whether or not an empty dungeon gets settled, and if so, by what:
1: Each week in-game, there is a 50% chance that something moves into a lair that the party has cleared out but not settled or otherwise secured. Flip a coin or something.
2: Decide on a tier. This is very much a D&D 4E term, but it's not rocket science. Regular wilderness is Heroic Tier. The Underdark is Paragon Tier. The wilder parts of the Planes is Epic Tier - it's where you start beating up demon lords, and maybe a god or two.
Alternatively, look at what lived in the dungeon before, and check where that belonged in the table in step 2. If the party defeated an army of Githyanki, it's a good bet that Paragon is about right.
3: Roll a d20, looking up the result in the correct column in the table to the right (click for a bigger picture). Since Epic is a bit anemic, reroll numbers with no entry.
4: Some entries like "Beasts" are very broad. If you want a more specific answer, roll a d10 (adding 10 or 20 for paragon or heroic tier), to see what level you should aim at.
So, let's try it. The party has defeated an adult red dragon, which is a paragon threat, from its volcano lair. They leave, a week passes, and the roll hints that it might be time for something to sneak in.
The d20 roll is 13, which means that a group of archons or elementals move in. We could stop here and decide that it's fire archons and fire elementals, but that's boring. A roll of 1d10+10 comes up a 12, and a quick peek in the DDI compendium show that a "Fire Archon Emberguard" is appropriate. Well okay then. Of course, other fire archons can play too, as well as other level-appropriate fire creatures - the d10 roll is just to point you to specific creatures.