Here is how I would do it:
Rather than the classic trope of a nuclear holocaust leading to the kinds of mutations one expects out of MF/GW, I would steer in the direction of Star Trek II: The Wrath of Khan. In other words, the thing that destroys civilization as we know it isn’t a nuclear conflagration, but rather the attempt to avoid such an event through genetic manipulation to create the perfect soldier. Thus, the end of the world comes with a "Gene War" rather than a nuclear one.
This scenario justifies the existence of elves, dwarves, halflings, beastmen and even mutants. All of these are attempts at super soldiers. It even justifies all kinds of cosmetic weirdness in human characters. What to play a blue-skinned warrior with a pink mohawk? Sure. Chalk it up to instability in the over-manufactured human genome.
An intro might look something like this:
Sometime in the future, the human race embarks on a journey to transform themselves by playing with their own genetic makeup. This coincided with the expansion of humanity into space. Inevitably, war happened. What the exact reason for fighting or who fought who is lost to time. In fact, no one rally even knows if they are over or still ongoing. Due to the genetic quest to find the perfect soldier, the conflicts that arose came to be known as the Gene Wars.
The planet that is now know as Ur is the site of a major space battle took place during the Gene Wars. Scores of damaged ships crash landed on Ur, bringing with them humanity, technology, and a menagerie of genetically engineered monsters. The space battle look place generations ago, and its significance is lost on all those who have descended from the survivors. However, the crash sites remain and there are still technologies and treasures of a much more advanced society still riddling the landscape.
Ur is now a planet of many races and monsters. Individuals from each generation rise up to brave the wilderness to find treasures and monsters at crash sites of the spaceships damaged from that long-forgotten battle.
I already have a collection of monsters I would deem MF/GW-worthy organized into three broad categories: Machine, Mutant and Alien. I would need to stat up some more classes/races: Mutants, Clones and Purebred Humans.
Thus, the final project in all this would include:
- MMII Elves
- MMII Dwarves
- Purebred Humans
In addition, it would contain all the monsters from SWCL, Swords & Shapeshifters, Fiends & Falchions as well as 30 more that fit in the MF/GW mold.
I can even imagine future/alien technology being described in terms of extant magic items from SWCL. For example:
Plasma Gun: Functions as a Magic Wand (Magic Missile usable 3 x a day); however, rather than being limited to the Magic-user class, it is usable by anyone but requires a Save to activate.
Grenades: Function as a Scroll, but (again) usable by anyone but requires a Save to activate. Thus, you could have a Stun Grenade (Hold Person spell).
Or, should I just do up another Alternate Version of SWCL that just encompasses MF/GW and let folks mix and match as they wish?