A playable mutant race would add a very cool aspect to the game. Maybe the mutant race tribal zones could only be set up in the green mist. This race would most likely attract evil aligned PK type players. Imagine a horde of grotesque mutants mounted on red-eyed mutant bears storming out from the green mist to raid a nearby human tribe.
There would, of course, need to be some drawbacks to being mutant or else everyone would want to play them. Maybe they get special mutant abilities but can't use certain technologies or craft certain items. They should probably also be auto-evil aligned so guards will KOS.
This is just an idea and not something I would expect to make it into the game until well after launch. I'm sure this would bring up a ton of balancing issues that would have to be carefully considered.