I loved Gamma World (GW) when I was younger, but when I started digging into the rules again to prepare for a new campaign, I found both things I liked, and things I didn’t. I was delighted to find so much fan-created material available for GW. I still like the wacky mutations and the concept of a world gone wild. I liked having three types of characters (Pure Strain Human (PSH), Mutated Animal (MA), and Mutated Humanoid (MH)), but I did not like the humanoid forms that Gamma World gave MAs all the time. I preferred animals that were animals, only smarter, so I kept the actual mass and abilities of the animals but gave MA an additional physical mutation, which they could use to add hands as a new body part (so they could manipulate weapons, etc.) or just to add another physical mutation to the mix. MAs have Packs or Herds, just as humans have Settlements or Bases. Mutated Plants (MP) and Synthetic creatures (Androids, Robots, Cyborgs) were not player characters in 1st edition GW, but as players in the 1980s, we often took them anyways, so I built them in. The Synths tend to walk alone, but the MPs have Forests or Patches. I still love the idea of Cryptic Alliances as ways of describing political & cultural priorities, though the original Alliances were heavily anthropocentric. After much work than I had planned on, I revised and expanded the original Alliances until they supported humans, animals, plants, and synths more equally.
Gamma World combat was very simple; too simple for me. I didn’t initially see the choices that the designers had made, which were actually quite good. I pulled it apart, understood, then put it back together but upside down, so I could have open-sided tables. So combat is pretty much the same. I don’t like the idea of Hit Points without Hit Locations, but to change that would mean changing the feel of the combat, so I didn’t touch it. I’ve never liked using levels, but really, that had more to do with the acquisition of hit points every level (something I’m not using here). So I adapted the skills I use in my Navah campaign and put them into a format that fits levels.
GW tech was fun, but had a bit too much whimsy for me. I wanted some straight tech to make the wacky stuff stand out, so I adapted the Traveller Classic equipment list and mapped Omega Tech to Traveller’s Tech Levels (TL). This gave me a much larger and more fluid list of equipment to both provide to players at the right time, and also to add to my treasure generator.
The original Gamma World had very specific origins of the characters and Cryptic Alliances, all tied to a national map but without a regional map. I abandoned most of the GW background and national map, and instead started building a series of Sectors (144 km on each side) that could create a regional and then national map as the characters adventured.
I also changed the cause of the mutations. GW used radiation, which evoked a 1950s idea of mutations. In Mutant World, radiation can spark a mutation, but not alone. Instead, the wild mutations of the world are caused by a bioweapon initially called Accelerated Progression & Evolution (APE). APE was originally designed to act only on certain controllable organisms, which could be easily eliminated once the enemy was overcome. Instead, APE turned out to interact with a popular herbicide in an unpredictable way, causing APE to itself transform into what became known as The Burn, an transformative virus that spread quickly and ubiquitously to most forms of life. Vast and devastating changes followed to animal and plant life, and the end of civilization as it was known. Millions died, a few successful sentient mutations survived. The PSHs of today are either the descendants of largely-immune ancestors or of Vault Dwellers who evaded the original Burn. As a result, PSHs only mutate if they fumble a roll after encountering either enough radiation to re-activate the dormant virus or when they find a rare patch of the original Burn, still active. Vault Dwellers, of course, mutate much more readily. Synths have often gained freedom in the Burn, as some of their control protocols were too specific to survive the death of their masters. Others still respect their ancient service, but only to PSHs identified as natural descendants of the original humans. Almost all Synths were reprogrammed to identify and reject control by those affected by the Burn.
Finally, I changed the radiation and poison rules slightly after two party members died from radiation poisoning. Not a big change, and they might have died anyways, but now they have a chance.
And that’s where Mutant World stands right now. There may be more change coming, but probably it will come as a result of the player characters more than me. Most of my design is done (I think).