In my soloplay with Castle of the Mad Archmage, I'm attempting to follow the restocking rules that +Joseph Bloch wrote into the mega-dungeon. But those come down to after awhile the creatures will be replaced by something similar*. That's pretty broad and leads us to the point of Hack & Slash's post. There are a couple ways you can approach it. You can come up with some sort of random stocking tables, such as a vermin table to determine what moves in after you kill the fire beetles in Room 27. It doesn't need to be limited to just insects and arachnids, any sort of non-intelligent creature would do. The second way is to have or steal a good idea and run with it. I'm in the midst of doing that. The party recently cleaned out the bandits on the surface, leaving me with the question 'What, besides more bandits, has a role similar to the bandit's role in he dungeon?' The first step is determining what the bandits' role is in the dungeon - stealing the characters
One of the esteemed Mr Bloch's rules is that dead characters have a chance of rising as undead. Not exactly the same as re-stocking as whatever killed them is probably still there, nut an idea that I shall shamelessly steal when running other dungeons.