we like to combine it with "Vault of the Fiends" (Heroic Level) because its alo dungeon-themed. When doing so, we often deal one more global quest (leaving the victory conditions at 3 quests) to reduce the effect that quest rooms tend to appear later in the game because of the double-sized map-deck.
You can also combine it with "Call of the Lich Lord" (Epic Level). But doing so the games overall length tends to be around 2-3 hours.
