Maybe the key to this is to just update and republish d20 Modern under a new name, with tweaks and adjustments found in Pathfinder and Starfinder to make it a new, modern d20 Modern. The rules are solid, though they could use some modification to fix the things that didn’t work as well as take ideas from newer games.
For instance, the combat maneuver mechanic was pretty bad in 3e and even 3.5. Since d20 Modern came out between those two, it looks like it had ideas they were trying out for the revised edition, but somethings from the old remained. The skill list was large, there were feats that were changed in future revisions, and the psionics rules were just horrible. One of the best things to come out of 3.5 was the Expanded Psionics Handbook that modified and changed a lot of the psionic rules and made them function much better and more streamlined. With Pathfinder coming out a few years later, those rules were further modified into the Dreamscarred Press book: Ultimate Psionics.
Starfinder also had some modifications to the d20 system, notably the KAC/EAC separation, but it also included things I’m not keen on like the addition of archetypes. They are better in Starfinder than they were in Pathfinder because archetypes can be applied to any class, but I still fear an ever growing list that changes how classes function, adding unneeded complexity and diversity to the point of over-abundance.
d20 Modern, however, is almost completely open content, so we can use it in its entirety. We can use the race setups from Pathfinder as a base since some of those races still exist in Xenomega, and I have even acquired a d20 Modern conversion of Moreau rules. That in itself needs some modification as well, but the base is solid. A little updating and changing and it could make the rules for creating almost any mutant animal imaginable.
The classes are also pretty good and generic enough while also allowing each character of the same class to have differences. Strong, Fast, Tough, Smart, Dedicated, and Charismatic. With some of the later addon books that came out, we have even more options for customizing the base classes without adding so many options that it drowns in them. The Talents system allows each character to choose their own abilities, while still remaining within the same class. The Future and some other books added more Talents, so even with all of those, they are still relatively small in choices.
d20 Modern also had Advanced and Prestige classes. In 3.5, prestige classes were almost required. Multiclassing was powerful and nearly needed to create a well rounded character. With the update to Pathfinder, that was scrapped as each class was built to make it more interesting to play all the way to 20th level. I still want to keep advanced and prestige classes though. Advanced classes are like specifications, focusing into a smaller subset of the base class, while prestige classes are things that allow characters to do absurd things and have access to greater powers. So to take a couple different base classes is required, going for a advanced class that narrows in on the character idea, but a prestige class is something lofty and nice, but not needed for everyone.
There are enough advanced classes out there, both in core and third party books, to fill a book on its own. I’ll start collecting all of those that seem relevant to the Xenomega campaign setting, probably copying all the text into one document to be added to the core book eventually or even make its own book completely of advanced and prestige classes.
For skills, I’ll take the basics of Pathfinder. There are a couple of skills that are in d20 Modern that don’t have counterparts in Pathfinder, like Computer Use and Drive/Pilot, but I can probably take those from Starfinder. Other things like reducing Hide/Move Silently into just Stealth will help reduce the amount of skills by a large margin. I don’t think there are really any other skills needed for a post-apocalypse setting that aren’t present in Pathfinder or Starfinder though, so that should make a rather robust list.
Then there are feats. There are a lot of feats out there. Some fit only in a magic-rich setting, while others can be applied to any setting. I will have to go through d20 Modern and Pathfinder to find duplicates and probably use the more recent incarnation. There are dozens of books out there though, many of them with twenty or more new feats, so going through that will take awhile and be a chore. I don’t want utterly useless feats or duplicate ideas, but I do want a lot of options.
For things like Magic, I’ll have to figure out how I want it to work in the world. I can take some of the core classes from Pathfinder and turn them into advanced classes, so we can have druids and witches and sorcerers. For divine magic, I’ll have to figure out religion and which classes best suit the nature of a modern priest. There may be archetypes in some Pathfinder book I don’t recall that will fit the setting better with a little modification, because I don’t want the typical cleric class running around and fighting monsters while also buffing and healing allies. There will probably be a few different cleric-type classes, like a cloistered monk who has studied religion and knows a few spells, but is much more comfortable as an orator and counselor. There will be a warrior-type priest as well, but likely more akin to a paladin and maybe several specific versions depending on the deity worshipped.
Psionics will also be advanced classes, so there will be a psion, soulknife, and wilder class. I don’t think I will have a psychic warrior though, as that can be accomplished with multiclassing a psion and some battle ready class. There are several other classes in Ultimate Psionics though, so I’ll have to go through that and figure out which ones fit the flavor of Xenomega and which ones do not.
Other things like technology are easy to handle. There will be cybernetics that can be purchased, but I’m thinking about using the electricity vulnerability that was included in Cybertech. Make it so there is a cost to having your body full of artificial parts. There are also biohacks, an idea I adapted from Dark Sun where the V’reen have genetically modified and created creatures that can be added to a humanoid body to add things like wings, extra limbs, external bio armor, or even extra muscles.
And lets not forget mutations. In Xenomega, there are both intentional and random mutations. The history of the world includes extreme genetic modification as well as the release of toxins and chemicals that can modify almost any genetic code. Some or good, some inconsequential, and some outright damaging. d20 Future and Apocalypse both have a large selection of mutations, so we can adapt those and add to them. There are also gene mods in Future, where the subject was intentionally modified to create a creature more adapted to a particular environment, and these will be kept. Xenomega has deliberate experimentation in it, so there are still scientists out there playing with DNA to create better soldiers or whatnot.
Later tonight, I’ll write up a table of contents based on d20 Modern for the Xenomega book. I’ll have to set up a file structure so I can keep track of where things are and what needs to be worked on. I want Xenomega to be its own complete game, so the GM needs only to buy one book to start playing. It’ll have rules for creating characters, running the game, as well as setting information. It will be far from complete though, so there will be room for more books that expand on the ideas and technology in the first book.