I've mentioned a few times that for mutant characters I use Darwin's World. I've had to tweak the system so that it works more generically than the post-apocalyptic setting it was designed for. Here's how I'm going about doing that.
First off, the default assumption is that most mutants with be human or near-human as a base. Alien species can have mutations, but in order to make use of the mutations the GM and player must come to a decision as to which default abilities are given up in lieu of the character being a mutant. (At least +2 worth of abilities for each mutation stage.
Mutant characters are broke down into 3 stages. The first stage are the weakest with the most defects with the third stage being the strongest with more major mutations and fewer defects, so no real change from the default found in DW. All mutant characters are considered Outsiders, which fits their background since most come from the worst hellholes in the galaxy. All mutations are determined randomly. The breakdown of mutation levels is the same as that found in DW with Major and Minor Mutations and Defects. I've even put together a table for Neural Mutations.
Speaking of Neural mutations, only the Novice level powers are allowed. Neural mutations do not automatically gain the Psionics Arcane Background, just three rolls to determine their powers and 10 power points. They can purchase the edge and become full psions, but until that point they are unable to advance their powers, and their powers cost +1 extra power point to use. (I tend to use the No Power Points rules for Psionics. In lieu of 10 power points, neural mutants suffer a penalty to their Psionics roll equal to the full cost of the power being activated.)