I'd be surprised if the interim game was a Phantasmagoria, as the previous two both occupy a whole number in the series. Impossible Spell Card looks to be a continuation of the Shoot the Bullet style as well, so I'd be surprised if a direct STB sequel was being made.
A Hopeless Masquerade expansion seems very likely (I seem to remember talk about character name data being found in the Hopeless Masquerade files, suggesting that an expansion was being planned before its release), a completely new fighting game less so. Which potentially leaves a third game set before TH15 (if not necessarily released before it). Aside from the Shoot the Bullet games, previous decimal entries in the series were deliberately marked as such due to being collaborations; for that reason, I'd speculate either:
- Another Touhou game spun off from a Touhou Manga, similarly to Fairy Wars, with art by the manga artist (a Wild and Horned Hermit game?)
- A collaboration with another doujin game group, either Tasofro or a different group entirely.
All speculation, but hey, speculation is fun. Of course there could well just be the one interim game before TH15, assuming TH15 happens. Hopefully it will. It would be weird for the series to end on a 14.8.