My basic idea for this is you are fighting on Black Forest or some other popular map from Fat Princess, with the red and blue armies fighting in the background. Partway through the match, an evil spell comes across the land, and gravestones start popping up out of the ground in the background, as well as zombies, which the red and blue armies proceed to fight. Meanwhile, on the playing field, Sir Dan's crypt rises from the middle of the map. Not sure if you should be able to go inside it or not.
Alternatively, you could be fighting in the Graveyard on Dan's Crypt already, and then Fat Princess soldiers come storming by with the princess, having random soldiers attack people in their wake. One of the siege towers from Fat Princess rises up on the side of the stage.
The second one I didn't think through as much, but eh, I like the first one better.
Tekken for one reason. Moonlit Wilderness from 5. I don't even play Tekken that much but that stage just screams epic showdown. And because it's so simple, a mashup with just about anything would be pretty simple to work in.
Currently Playing: Playstation All-Stars Battle Royale, Infamous 2