First is that the franchise is a lot more experimental with its mechanics. Digital Devil Saga is like a traditional jrpg without demon negotiation or fusion, Persona is a dating sim with simplified SMT mechanics bolted on top and the Raidou games throw real time action into the mix. Even the mainline games have radically different mechanics from title to title, with Nocturne being radically different from SMT 1+2 and IV+Apocalypse being radically different from Nocturne. You could probably take any of those titles and adapt any one of them, but you can't really adapt all of them at once without taking some creative liberties.
Second is that the mechanics that are common to most series are extremely videogamey or simply unfeasible in a co-op multiplayer environment. Press turns are a great JRPG turn-based system but only really work when one player = one team and having 4+ teams running around, each using press turns, would be a huge mess. Demon fusion is flat out impossible to translate in a way that doesn't involve tons of charts and spreadsheets while still retaining what makes it interesting. Giving all PCs a rotating roster of demons that constantly changes as the result of negotiation, fusion and plot means having a ton of insignificant NPCs running around instead of a handful of meaningful ones. Point here is that it's not just that most mechanics are different from game to game, but the ones that tend to stick between games aren't the kind of thing you actually want in a tabletop game.
Do note that I already knew this was going to be a system where most PCs would control and fight with monsters, which was already very difficult to adapt to the tabletop RPG experience while keeping it elegant. Having a system like that that wasn't Pokemon was the entire reason that I set out to make the game in the first place, but I certainly didn't need to make it more complicated than it already was.
What about instead designing around the aesthetics, worldbuilding and general tone of the franchise? Those also change a lot from game to game (purists still argue that Persona is not a 'real' SMT game, after all), but if I cherry-picked a handful of them I could adapt them to a pen and paper RPG, which is more than I could do for the mechanics.
Here's the List of SMT Things That I Decided To Focus Around:
1) Mythology Kitchen Sink
The games feature tons of mythological characters in it, ranging from the well-known to the obscure, but they're not just window-dressing. You can reasonably expect accurate lore from them, which the plot is written around. Satan is a different character from and enemy of Lucifer, Shiva is a god of destruction and rebirth rather than a lady with ice powers, Loki is a trickster instrumental in the fall of the otherwise invincible and immortal Baldur. Mixing so many contradictory mythologies together means sacrificing some fidelity, yes, but so does telling a good story where the PCs get to do anything of relevance and the plot isn't already written in stone. I wanted to cram as much mythology into the game as possible, so I figured I'd go for five factions instead of two or three, which leads us right to...
2) Moral Extremism
In SMT the factions are extremist assholes. The demons, too, are extremist assholes. Even your friends are extremist assholes! The commonly agreed best ending in most games is not allying with anyone but murdering all of them, because most of the people vying for power in these games are up to nothing good. Usually the conflict is between Law (aka God) and Chaos (aka Lucifer), but if you've ever played D&D you know that the Law/Chaos divide isn't a particularly consistent one, so I would have factions focusing on more concrete goals. I have the WAR! faction the SCIENCE! faction the NATURE! faction the ROBOTS! faction and the FUCK
3) Grimcamp Edgebright Tone
The games head into pretty dark territory. It doesn't stop at killing as a way to solve problems, it usually involves brainwashing, slavery and flat out genocide as things that players have to stop or can choose to support. Still, there's moments of levity, from dancing with Mr. Thriller in hell to Charon telling you to go take a number and whining about the queue when you get a game over. The shifts in tone can be very quick: one moment you're watching a teenager try to kill herself to escape a sex offender and the other you're fighting a giant penis demon. First you're dramatically shooting yourself in the head to summon a demon, later you're putting a shock collar on a dog so he can do the same. The tone is only consistent in its inconsistency. Between snarky commentary from named NPCs in most sections and completely ridiculous shit being treated with stone cold seriousness (such as everything about Orgone), I think I've got the tone right.
4) Negotiation Matters
Demon Negotiation is one of the highlights of these games, the quirky conversations you can have with demons (or that your demons can have with other demons) are a big draw into the setting, but the inherent randomness in the system is often a turn off. It is also one of the most frustrating things about the franchise, because you have a bunch of games where talking to others is an actual game mechanic, yet significant enemies simply refuse to do so. I wanted to make Negotiations a big deal in Monsterpunk, so I figured that instead of making one PC the dedicated face of the team everyone would have a Talky PC Skill they could use. Negotiations are relevant in combat not just as a way to finish things but also as a way to weaken enemies, they're a thing that anyone can do but that still feels unique for each PC. I'm quite proud of how this one turned out.
5) Making Pacts for Power
Many of the games begin with the main character signing a contract with an entity, this usually saves their lives when they're in danger thus ending the tutorial. Monsterpunk pretty much takes it for granted that this is going to be the origin story of most PCs. I decided there would be some pure human classes focusing on technology instead of making pacts with monsters early on to add some variety, but otherwise this is the heart of the game and the thing that makes most of its PCs stand out. It being last in the list does not mean it being least in importance.
And I think that's all. What started as making a Megaten RPG ended up being a Megaten-flavored RPG that is better described as "Devilman meets Fallout" instead. BCG is clearly and blatantly SRW, but Monsterpunk is not Megaten. Is that better? Worse? That's not for me to decide.
Next: Changes! What things I'm thinking of putting on the chopping block.
Gimmick Out.