Curate, connect, and discover
I'm gonna get straight to the point since I don't really need a big post since the theory is self-explanatory
Hanako of the opera AU ends with Kou and Mitsuba basically running away from the opera (or to be fair, Kou kidnapping Mitsubađź’€)
Then we get this in chapter 118, which is basically Mitsuba saying, "Hey my platonic friend, let's run away together in a platonic way so that we can forget that we're doomed asf(platonically)".
The panel shows a train. They could've drawn snacks or silly stuff, but they chose a train?? What's the only other time a train has shown up in tbhk?
To finish off, Kou has been hinted at becoming a supernatural multiple times. One of my favorite examples is the Ghost Hotel AU..
...it's canon that in this AU exorcists are still a thing, because Teru is one...but Kou isn't one? And what does Teru say he's doing in this AU? Searching for his little brother.
Honestly, seems like foreshadowing lol
(And btw, as a fandom, we ignore HOTO way too much)