I feel like they need someone to ntr Senpai.
It would be something like this: Senpai doesn't confess to Nagatoro, Nagatoro gets frustrated over it, the ntr characters comes to her and convinces her to have sex, she has revenge sex with him but regrets it, ntr characters sends a video of the sex to Senpai, Senpai realizes this only happened because he didn't man up and confessed to her, and he feels the need to rescue her from the ntr character.
He comes to her and confess all his feelings, and she tells him she also likes him but she is pregnant.
He decides to marry her and raise the baby together with Nagatoro.
Happy ending.