I still wonder about how Wakana, Osako, Rin, and Otoha are going to join the harem though, I expect it to be well written, and not just as a "everyone miraculously falls in love with MC" like a lot of other harem games do it, Selebus is an amazing writer after all.
Rin has already confessed that she's really into Sensei, she's just shoving that off to the side right now because she prefers girls and Otoha didn't say no. If anything happens to break Rin's heart, falling into Sensei's arms is pretty much a given.
Wakana's also not that hard to see hooking up with Sensei. Osako kind of makes it clear that she is worried that Sensei can pull Wakana away from her, which would imply that Wakana's either bi, or something like a sapiosexual (attracted to minds rather than genders) or gray ace. Sensei's got enough knowledge of poetry to appeal to her intellectually (hence Osako's concern), and with Imari pulling them both to happy hours, there are going to be more opportunities for them to to get to know each other and for that to develop into something. Either that, or Imari gets them both drunk and convinces them to have a threesome. I wouldn't put it past her.
Otoha is trickier. Either Rin's attraction to Sensei becomes an issue and Otoha agrees to an ill-advised threesome to try and resolve the tension, or something else that would seem very....unusual. At the moment, she doesn't really seem to see him as a guy, and her alarm at the idea of him sleeping with Chika was such that I think she'll be one of the last ones to hook up with him.
Osako -- the only way I see this working is if Wakana doms her into it. We've seen that when Wakana is in dom node, Osako is powerless.
I think it's a good time to repost this post from last year, back when Selebus was still cruising around here. It's a general roadmap of the game's trajectory. He's indicated that we're about %20-25 of the way through, so we'll probably see the Dark Route either in Chapter 4, or at the end of Chapter 4 (I suspect the Chapter 4 cliffhanger will kick it off fully, with Chapter 4 itself feeling half Harem/Half Dark)