Diablo 3: Hardcore mode. It would be next to impossible to implement in this game, but what about one wipe instead, after which the group is locked out of the instance for 24 hours? It'd definitely be a challenge then :O It could also make dungeons like Castrum Meridium and Praetorium less boring as well.

Romance of the Three Kingdoms VII+: Debate battles. Kind of an amusing type of "combat", especially for healer classes and rogues/ninjas (this last one will probably only make sense to me). It could be an alternative way to win battles, by persuading your way to victory, rather than just brute forcing your way to such. It could also make NPC interaction more, well, interactive as well, and not just skipping all the quest text.