I think my biggest gripe with the series aside from the sisterfucking part is the bullshit in the anime.
Why did she brainwash them? We can see she's a manipulative little shit so why didn't she convince them to be on her side? For some of them I can get why it would be hard like Ibuki and Mahiru, but some of them would have a lot of promise. Teruteru joining to protect his siblings and to make sure no one dies of the disease his mom has, Fuyuhiko and Peko trying to figure out who killed Natsumi, Hiyoko not having anyone on her side, etc. It would give them some sort of depth and add to the development of them.
If they wanted to do a sequel, Make it V2 so we can get Rantaro and Tsumugi actually developed and not rely on vague bullshit at best.
Also gonta did absolutely nothing wrong.