Remember kids, never talk to strangers. Or ghosts.
So now we have confirmation Michi is experiencing gaps in her memory with at least 3 confirmed incidents (her first meeting with Kasumi, Sayaka seeing her talking with Aizawa, the summer festival). From the way she was acting at the end, this could potentially be a 4th incident of her memory getting wiped? Maybe her younger self(?) is responsible? Either an overprotective guardian, or maybe something not so neatly good or evil. Was she the one that blocked Michi from talking to Aizawa, or was it something on Aizawa's side?
I'm so excited to see the mysteries taking off; I never thought it'd run this deep when I picked it up. All I knew I was getting into was a silly manga about a dumb gay ghost. We now have 3 major overarching mysteries: Why did Aizawa die/what makes her unique from other ghosts? Why does Michi have an automated defense system that won't allow her to speak with Aizawa? And what do the marks mean, and how do they bind these two together? Two literal star-crossed lovers: now we've got a story.
Random other details: The calendar says October. There's a flier for the Koga (古賀) Festival, which is a city in Fukuoka. Sayaka drew an odd shadow over Aizawa which I'm not about to overlook. Kasumi and Sayaka are the cutest couple. I'm getting more curious why Michi's mom isn't around, considering it's a story about ghosts. With memory loss, it's also now possible Michi was the one who made the mark binding Aizawa in the first place. Given all the emphasis on how childlike Aizawa is, is she missing memories too? Does this explain Sayaka's density, or was she just born perfect?
And here's some heavy speculation. There's no direct evidence, but the possibility's open, and it's a strong possibility...
It's October. The summer festival would have been about ~2 months ago. Michi can't remember what happened that night.
Aizawa died ~2 months ago. Something horrifying?
Edit: I forgot that, according to ch4, Aizawa canonically died on August 20th. That would put her death right around Obon, one of Japan's biggest festivals honoring the dead, when spirits are meant to appear.