Alright, seeing as how Touhou 20 is probably going to be released in 2025 (given the release dates from historical data), I've come up with some various theories and predictions for TH 20 and other releases based on previous games.
This will include spoilers as it will talk about previous games and the stories behind them.
This is also for fun! So you can take it as is or disregard it as you like as this will include my own supported viewpoints to the series.
You guys can also add some predictions for what you think.
1. Touhou 20 | Will Focus On A New Plotline / Develops The Beast Realm Plot
Alright, this prediction requires some backstory of the Touhou games, as people who know about Touhou lore know that a lot of the games are interconnected and sometimes reference past incidents and/or plotlines (EX: the current Beast Realm plotline and/or involvement of characters from the last game as seen from 6, 7, and 8, though many games do use past characters who aren't often seen as MCs).
Introduction
To start off, Touhou 20 is probably going to be a very important entry to the series due to previous games such as Touhou 10 being a new introduction to various characters and also Sanae, who has been a constant in the current Touhou games as a main character, aside from a new plotline given Kanako influencing Touhou 11's main plot. That aside, I feel like ZUN would probably be more inclined to make Touhou 20 a special entry that would either start another incident or continue the Beast Realm plot.
New Plotline / Continuation of Beast Realm Plot
This prediction is based on the length of previous plotlines and the focus of making Touhou 20 into something newer in the multiple entries. As the title says, Touhou 20 will probably involve a new plotline aside from the beast realm, which will probably be of some importance, but not enough to make the beast realm the main focus of this entry; it can also be put aside for another new incident.
The other theory means the continuation of the beast realm plotline given that Touhou 19 UDoALG still has the three leaders kind of agitated against each other for control of Gensokyo, though the data to back this up is pretty low considering Zanmu's UDoALG ending has all three leaders of the beast realm convene and make up with Reimu acting as the peacekeeper, though the beast realm will probably still be of relevancy if brought up again.
So to the new plotline I have some generalities that might be expected
1. Eastern thematic: Touhou has constantly focused on using characters from eastern mythologies for different plotlines; the use of western themes for a main game plot hasn't been used since EOSD, which focused on vampires, witches, maids, demons, and fairies. (To which I feel kind of disappointed as some character designs are from western backgrounds and fangames have provided great explorations of how it could be done for the main plot (BDiTB).)
2. 3–4 playable characters or just 2 main characters: This one needs more explanation as many people who have played Touhou know that many of the recent entries do include more than the usual two, though with the current theory "New Plotline," this might be relegated to just Reimu and Marisa with either 2-3 shot types that will lead to different endings. Otherwise, 3–4 playable characters will be the mainstay, given the recent entries which are probably the most likely.
3. *Something more modern*: This prediction is kind of more loose, so you can disregard it more easily, but I'll still talk about it anyway. Seeing as to how Gensokyo is constantly facing off more and more threats that have some theming around modern-day stuff, I feel like Gensokyo might face off with another kind of vague relation to the outside world that isn't entirely another character, but something that incites a new character into another incident.
2. New Touhou Fighting Game
It has been about ~6 years since we had a multiplayer Touhou fighting game (disregarding GI and UDoALG as they don't fit what I'm actually talking about; the last one was AOCF in 2017). Studio Tasofro's recent work, GI (Goukyou Ibun), also called on the TH wiki Sunken Fossil World, has been kind of in production hell as it was supposed to release after WBaWC (presumably due to the effect of COVID-19 affecting the production schedule and release date).
This wasn't exactly what people most expected from the game, as most people here felt that the gameplay fell short, though the music and art style always hit with people, given that Tasofro is made up of popular producers of doujin games.
So maybe within the next year or two, there might be a new Touhou fighting game, as it is 2024, and Tasofro's regular schedule of fighting games might come back.
3. New Side Game
Kind of expected given the release timeline, what type of side game is probably going to be dubious as most of the previous side games are either mostly STBs(Shoot The Bullet) and or one of the prominent ones GFW or 100th BM. Though one thing is for certain, it's going to include bullet curtain mechanics as it seems that is the only thing ZUN knows how to program a game around.