Just an idea I had, since I've been playing League of Legends. How about a world that has dedicated arenas, with champions fighting each other? Have the game actually designed for it, too. I'm thinking something similar to d6 or the Supers! rpg would work well. Heck, this is one area I think 4e D&D would actually shine. I'm not sure old school D&D would do as well.
Instead of having monsters, it's just NPC Champions to fight. Team vs. Team, or even One vs. One. Heck, powerful monsters would be champs, but actually written up with a name and history.
This would also allow different arenas. Maybe one is all ice, with big
chunks of snow and clear ice sheets that block ranged attacks. Another
could be a jungle, and then just a flat plain for some matches.