When it comes to video games, a good fashioned horde mode makes for a lot of replay ability, at least for me. That was the main reason I enjoyed games like Left 4 Dead, Gears of War, and certain Call of Duty games. There could be MANY ideas and ways to branch off with the idea of a horde mode. The main ideas I was thinking of were: Achievements for getting to a very high wave, Coins based on the number of creatures killed and the difficulty of said creatures, Special equipment, etc.
When I think of Horde Mode I think of just an endless number of battles where it gets tougher and tougher, starting from something small such as a couple of mud crabs, but then gets tougher and tougher to where you're stuck fighting minotaurs, lich, cliff racers, necromancers, guards, bandits, maybe even dragons and various npcs that are otherwise marked as essential.
Would you like a horde mode?