I have a suspicion that the next game will be a 'Phantasmagoria of'—type game.
If you've paid attention you will notice there's a pattern to the releases and their features:
——Hopeless Masquerade featured an Extra(you access Phantasm from Extra, okay?) boss as the penultimate opponent, just like in IaMP.
——The game right after was a shooter with a motif based on a Japanese story (Kaguya-hime and Isshun Boushi, respectively).

This implies a split-screen shooter will be next, followed by the newest Bunkachou.

Heres Hopeing, I loved Of Flower view and I will definitely buy the new one.

I really, really hope so as well. I almost like that style of game more than the usual~

I always liked the phantasmagoria game(s), even if I never played the first (more than a couple of stages. Those pc-98 games are hard). The stories are always great cause they focus on lesser characters and make them all a bit deeper.

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I hope for a position of playable Sekibanki and Tojiko.

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You'll get used to the differences.

Oh, I just realized something for furhter speculation. Touhou 7 and 13 both involved ghosts, where both Youmu and Yuyuko appeared. Plus the extra/phantasm bosses were really really old XD

If touhou 15 will be a phantasmagoria game, what characters do you think will appear? Sekibanki would be a really great one, she's awesome!
The games stretch back all the way to 10... so Nitori might well appear, Satori.... might be a stretch, unless the plot reaches all the way to the old Hell...
Kokoro would be interesting to see again as well, she was adorable.
Plus, the last phantasmagoria game gave back a pc-98 character, Yuuka, who still terrifies me to this day. So it might always be possible to see a Mima, Shinki or Elly appear, though Shinki is probably a bit odd as she has a big position, unlike Yuuka did...

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Good analysis.

There, out in the darkness
A fugitive running
Fallen from god
Fallen from grace

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An angel, right?

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Yuuka'd better show up~ Sanae, Rin(?) Yamame, stage 5 and 1-2 bosses are a decent bet.

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The only angel I know is Sariel... even though my memory seems to say there's another 1 in the pc-98 games.

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Sanae! How could I forget! I still can't believe she was absent in Hopeless Masquerade and Double Dealing Character. And stage 4 bosses aren't too out there actually, considering Lyrica had a story mode in the last with Merlin and Lunasa playable too. Though there are other stage 4 musical youkai now...
I also have to admit, I'm really aching to see Kanako playable or at least appearing anywhere again, alongside Mokou, Eirin and Kaguya they all seem to never appear despite being great characters.

And of course there will be original youkai as well. I wonder if any other tools and instruments were brought to life from the last events.

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Shinki is—or rather, was an angel.
Sanae isn't absent from HM.

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I'd love to see Kanako as well, but there aren't many stage 6 bosses in PoFV~

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Background characters don't count

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I see then...

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Any ideas what the recurring theme will be?
We've been through the general presentation of Gensokyo and then the faiths so far.

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Hmmm, perhaps a resurgence of the underground youkai~? I really couldn't begin to guess~

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Maybe.
Or perhaps it's something similar?
The rise and fall of a society, politics and reigns?

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Hmmm, well PoFV turned out to have a surprisingly deep story, so maybe~

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It did.

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Well here's hoping for a civil war in Gensokyo

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...That already happened in DDC.

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Well~
PoFV had one of the ten Yama as final boss, which as far as Gensokyo is concerned is one of the highest authorities, but also not from Gensokyo itself. PoDD had someone from outside Gensokyo as well...
Maybe we will get a new visitor! Maybe Science will have an uprising in it!

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The return of Rika(ko)?

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Probably not as the big youkai, but it's always possible in the lands where fantasy is real!

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I think one of them will be the Stage 8 opponent if it isn't Mima. Perhaps even both.

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Oh, I really hope they bring back Yumemi~ She's definitely my second favorite Oldhou, rockin' Gensokyo with the power of science, na mean~

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Not to mention she's the only purely human final boss save Reimu in the series.

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Technically, Mokou is considered human, or she was once human atleast.

Here's some other interesting bits
Sakuya was playable in PCB and DDC, excluding IN since there were a lot of other appearances there and they worked in teams. Cirno appeared in both these games as the first stage midboss as well. Stage 2 takes place near a town... Stage 4 is in the sky and features musical Youkai... If we could believe these things then Touhou 15 may well be another IN. And yes, though IN was based on a tale that was natively japanese, Yuyuko herself is also based on a natively japanese tale, so this can still be applied.
Just another side to the speculations, it can of course go any way

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But she's a bonus boss!

That might be right.

In the end I think comparing PCB and IN with TD and DDC not per game, but as groups, you can make a looooot more connections than you otherwise could. So let me see what I can get on that!

The themes:
The general themes seem to be somewhat diverse, and there isn't really a comparison in these (like PCB being spring/winter related)
There is however a theme around spirits that PCB and TD share.
The characters in PCB and DDC are the same, with each having two shot types. In IN, you have four teams, and in TD you have four characters, with only Sanae replacing Sakuya, but Youmu still playable, while you don't get a shot type you can really choose (excluding selecting just one character instead of a team in IN).

Stage 1:
In both PCB and DDC, Cirno appears as the stage 1 midboss. Stage 1 is generally not related to the plot, so there isn't much else to compare here

Stage 2:
Chen in PCB is a cat, while Kyouko in TD is a dog. It feels like these two share a bit of similarity as not being completely plot related second stage bosses. Besides that, stage 2 of PCB takes place in a village while DDC takes place near a village.

Stage 3
PCB and TD both have a returning character appear as the midboss (and boss in Alice's case) of the stage, Alice and Kogasa.
IN and DDC both have a half beast youkai that gets stronger under the full moon, Keine and Kagerou.
On a side note, stage 3 of DDC takes place in the bamboo forest of the lost, obviously related to IN.
Alice and Kagerou both direct the main heroine to the sky for stage 4.

Stage 4:
PCB and DDC both have the stage take place in the sky, where the enemies are musical youkai. In PCB, the one that fights you first and greets you depends on your character, while in DDC it depends on your shot type what order you meet them in. And both groups fight you for fun or their own reasons, and direct you to the big bad's hideout.
IN has a very unique stage 4, and TD isn't comparable with any of the other three.

Stage 5:
These are always in the hideout of the big bad and have their right hand character as boss, so this isn't really relevant to compare. The only comparison is that the stage 5 midboss in TD is a ghost, like Youmu in PCB.

Stage 6:
Oh boy, this is where it gets fun. Miko and Yuyuko are both based on a historical character, while Kaguya and (I don't know the DDC final boss name that well yet) are based on japanese folklore. Yuyuko is a ghost, so dead, while Miko underwent a resurrection, so both have a relation to death. Yuyuko of course also later appeared in TD in the first stage.

Extra/Phantasm:
Not counting Phantasm as something seperate, considering its unique, the first notable thing is that all extra stage midbosses are returning characters, with Ran and Neu being closest for being Extra stage bosses and Extra/Phantasm stage midbosses as well.
Yukari and Mamizou are both very old and knowledgeable youkai, that both seem to know how to have some fun on other's behalves (mostly shown for Mamizou in HM). Yukari and Mamizou are also slightly related to the plot, as Yukari was the original cause while Mamizou was already called in by other Youkai. Mokou and Raiko on the other hand seem more like extra chapters, where the incident is already resolved and you get an extra piece of fighting.

And finally, both games had a fighting game between them. While HM wasn't the first fighting game, it was a completely new concept compared to the previous three, almost like a bit of a new fighting game, while IamP was the first actual fighter. Yukari and Mamizou are more related here too, as they are both always the second to last boss except for their own stories, and both know really well what is going on while letting others deal with it.

Phew, now that was a lot of comparing, and there are probably a lot of things I didn't catch. But all in all, I find that the four games are more related as sets than as individual games, which would of course imply that Touhou 15 might be a phantasmagoria game again. But feel free to add more as I have yet to look at topics like music or dialogue.

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Kyouko is a yamabiko, not a dog.

The name of the kobito is Sukuna Shinmyoumaru— or Shinmyoumaru Sukuna for westerners.

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Also, good job.

The fact that Seija is still at large is an interesting factor for the prospective games

Well, what do you know. It was a factor.

3810873 what

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Impossible Spell Card.

With the new album involving cause and effect, I can only imagine the plot will be of fair consequence. Maybe some investigation of Gensokyo will happen?