I really like the brainstorming made by the people about how the story will evolve. Then again, the complete picture is in the DPC and it's difficult to make it out.
I also mentioned this in one of my posts, about how the antagonists drive the story forward. In the first episodes, Quinn did this job pretty well. There were a lot of unknowns and it made the story progression very interesting. But like you said, now Quinn is becoming a love interest in the DIK route, and even in the Chick route MC's relationship with her kind of softens a little (MC was calling her evil, if you speak with Maya).
If there is going to be a bad ending, it has to be only one and has to be unrelated to the branch we take. Otherwise, it is an almost impossible task at this point to create bad endings for every possible LI's branch like
mayrrrrrain mentioned. The one option that seems feasible for me is the police involvement with Burke's actions and prostitution/drug activity which has a connection to some students like Quinn, Lily (or Vinny), like
yossa999's theory. Depending on the MC's decisions with these characters, it may lead to a bad ending.
In the current situation, as
walker188 mentioned, the story does not have any strong antagonists. I think DPC needs to spice things up, we need an escalation event that MC needs to fight with. Maybe some secret antagonist may show up (like Acting Lessons). But as the story branches with different LI's and events, creating a generic escalation event for all possible branches gets difficult. I hope DPC has something planned for us.