Until now the possible "happy" endings seemed to be:
1. Sasaki getting senpai, leaving Nanasawa to eventually time slip and create a closed loop
2. Nanasawa getting senpai, rewriting reality and removing Sasaki from existence
But now we have a better option for everyone:
3. Polycule. The only true solution for a love triangle
I think Sasaki is gonna end up going through time again
I don't know if I'm the only one who thinks this, but in my opinion it's pretty obvious that Kaimori likes Nanasawa. I mean, I see a fuckton of hints. Like the author is screaming it with a megaphone.
My perfect ending would be Sasaki gets Seto, and then Kaimori gets Nanasawa -- after confessing to her that she always loved her but thought she had no chance because of Seto, and thus decided she would be unselfish and help her get Seto instead.