I'm thinking of changing the game system for the Xhuul setting from Gamma World to Castles & Crusades. One reason for this is that it would allow me to use the stable of monsters from the various Monster Manuals, Fiend Folios etc., as creatures. Also, I can use the existing spells in the Player's Handbook and other supplements (of course ditching the Vancian spell-casting in lieu of Power Point Costs of casting spells and keeping my alternate Spell names for flavor). Magic items can be used readily and if the object is a technological artifact, it's basically a reskinned magic-item.
This of course means necessary changes to many of the Mutations powers from Gamma World, so that it is in balance with the lower hit point values. One monster with the Life Leech power as currently written for Gamma World would wipe out a 1st Level adventuring party by the 2nd round of combat (assuming all were within the area of effect radius of the power). And that power in the hands of a PC would just be too damn powerful as written. So changes need to be made.
I haven't worked out all the details as yet, but I'm pondering the only classes would be Fighter, Magic-User (renamed as Sorcerer), Rogue and maybe Barbarian. I will add a new class called Mutant. The only race in the game would be Humans.