Real talk, I have a google doc that's over 100 pages on how TMS 2 could function and what I'd like to see from it. The bulk of it is information on playable characters (class, Carnage spirits, aesthetic and inspiration, song style, notes and biography for supports, etc.), NPCs, and antagonists. But gameplay pitches, dungeon themes and bosses found in them, recruitment time and the calendar system,* several major sidequests, post-game content notes, NG+ notes, shops and locations that can be visited, and several more things are also included. I also have several side documents to parse out some of the more granular information and ideas, such as affinities and how they work into gameplay. I've worked on this on-and-off since 2020, so it makes sense that it's this long. (The bulk of the work was done between 2020 and 2022, with some major revisions in 2023 and only minor touch-ups this year.) Maybe someday I'll commission some concept art of the idea and be foolhardy enough to pitch it to Nintendo or IntSys, but not today...
My notes for TMS 3 & potentially 4 are far shorter, roughly 6-10 pages total.** My notes for a true SMT x FE crossover are probably 3 pages at most. (My notes for an SMT x FE are a single page, and I'm pretty sure I've combined several of the ideas I wanted to try out there into another game or into SMT x FE proper.)
*Unlike Persona and Three Houses, tackling the non-story dungeons will not affect the calendar at all. There's an in-world reason for this too!
**The big problem is that I'm not sure what I want to do with TMS 4. Originally it was its own story, but then I worked it into TMS 3 as "The B-Sides," a story that takes place halfway through the game while the main plot is happening, that has its own story, characters, and gameplay style. But then the theme I wanted to explore with this story changed, and I found another way it can work into TMS 3, but I'm not exactly sure how it meshes altogether just yet. Ultimately, TMS 3's story, theme, characters, style, and key aesthetic are all set in stone for me, and have been for the longest time. TMS 4's key aesthetic changes due to two competing ideas, both of which could fit in TMS 3, but not together. Well, maybe together, but it'd be weird. I'd have to think on it.