One thing I really enjoy doing in my homebrew campaigns is monster theme-ing on the dungeon/regional level. It's helpful on a couple of levels, from giving players consistency they can prepare for to giving dungeons/regions a distinguishing character.
So here's a list of monster themes. I'm trying to go a bit beyond the typical ones like Undead and Fungi, as they're used fairly often.
1. Really BIG monsters. House-sized minimum.
2. Sorcerers with very specific powers, immunities, and weaknesses. They hate each other.
3. Cronenberg, but not from a human-centric perspective.
4. Things that used to be dynamic, but aren't anymore.
5. Things that weren't dynamic, but now are.
6. Planescape-style philosophically oriented factions. With very big guns. Too big for their own good.
7. Megaman style baddies. Each unique. Often ridiculous.
8. Enforcers of historical versions of abstract ideas such as Justice and Freedom.
9. Much smaller, weaker, and numerous. And also smarter.
10. Strategically oriented with little self preservation.
12. Non-sentient things that are now sentient and can communicate.
13. Language puzzles in monster form
14. Indefinitely duplicating things triggered by specific occurrences.
15. Clearly better than the party in every way, man for man. Now you are the goblins! (This one's pretty common, actually)
16. Everything has illusion magic.
17. Weather powers. Weird weather powers. State changes based on weather.
18. Musical monsters.
19. 5 Man Bands, or Power Rangers styled groups.
20. Cute but deadly. But cute!.. But deadly.
It's all a bit strange, now that I think about it. Monster themes as opposed to space themes. People usually design the idea of the character of the space first, and then occupy it with monsters, as opposed to the other way around. It gets me thinking about the possibilities of the vice versa, of designing spaces around monsters instead of monsters around spaces. How do these types of adventure spaces differ? How does it change adventure design?