After getting the Midnight expansion, I discovered that the two sets of heroes really weren't that compatible for playing a mixed game (you couldn't put all of the heroes into one big pile for everyone to chose from, regardless of whether you were playing the expansion or not.)
Do the other two expansions allow you to effortlessly mix two sets of heroes together in one fashion or another (expansion play only or core game play only?)
For me, the coolest would be to allow for a large library of heroes from the core game and two expansions that could be used in a variation of the game.