I would only want 2 guest characters for MK, both console exclusive.

Nariko for PS3
Frank West for Xbox

Here's why:

Nariko hasn't been in a rated M game but I think she would fit perfectly. It also helps that her universe has paganistic beliefs, so the Elder Gods could easily exist in her universe. Nariko also could be a really unique and fun character. Her sword can be used in 3 different ways: Speed, Power, and Range. So she could have really interesting gameplay.

Frank West has been in a rated M game and disembowled many zombies. So he definitely fits as far as the violence aspect goes. Frank could have a ton of special moves ranging from throwing CDs to doing Knee drops and Face Crushers. As far as story, Frank is always looking for a scoop, so where there is a story, there's Frank West.