Caverna: The Forgotten Folk
Buy this firstBGG rates it 8.22 from 1,671 ratings, the best of the large expansions. One BGG poster says it adds interesting twists while the game stays fun.
Needs: Just the base game
Adds: Eight species or race packs (elves, dark elves, trolls and more) that each player can play instead of dwarves, each with four unique rooms that replace base-game rooms.