DPC probably left us breadcrumb info at the end of EP3.
MC's thoughts:
MC: "Doors are funny."
MC: "Be it metal or be it wood, they’re made to do the same things."
MC: "To keep you safe or secluded..." (Thank goodness Troy leaves the room in EP1)
MC: "...and to protect the things you value." (The door did not protect the MC's guitar)
MC: "Protect you from whatever is on the other side." (We know who is on the other side: Josy. But the MC calms down again in EP4)
MC: "Or worse..." (Beginning of info about Isabella's door.)
MC: "They are made to keep things on the inside." (Seems to be about an object or objects)
MC: "Things you don’t want anyone else to see." (Jill doesn't seem to know what's in the room. I assume the room was not locked before the MC arrived at the college, but Isabella is hiding something there that has to do with the MC that no one, not even Jill, gets to see)
MC: "Locked away." (Okay, so we know Isabella locked the door)
MC: "It’s weird to think that something so simple as a knock, often repeated three times, is all it takes to get someone to open that door." (Here DPC makes a connection between EP3 and EP5. The MC knocks three times on the locked door in Isabella's house and Isabella responds with a startled expression)
MC: "Curiosity takes over and it exposes you." (A very important point in the context of EP2 and EP5. In EP2, the Freeroam event ends with the MC trying to open the locked door. In the Free Roam event of EP5, the MC can see under that door. Curiosity takes over and exposes you. Isabella will realize that the MC lied in EP2 and remember everything he did before he went to bed. The MC makes a noise as he tries to open the door and then lies down on the floor. Isabella is in the kitchen and may have heard this uand quickly checked. A DPC trap that I can't yet see where it leads)
MC: "Some doors are meant to stay closed." (This suddenly sounds much creepier than before)
MC: "Because opening them..." (Somehow this implies that we ourselves determine whether that door is opened by our play or by a direct decision. The strange thing is that the MC is already opening a door that could have negative consequences because we see a man's sacko. Maya's closet door)
MC: "...may change everything." (Sounds to me like if this door is opened, the MC's past and future life will change completely)