I've got some other predictions of my own. If they do make a final floor (or a Chapter 4), it's gonna be one of two things. It's either gonna be a small prep floor like Blue Womb or Home, or it will be something akin to the Mirror Realm from Downpour combined with the Mirror Rooms, but it's an entire floor. Also predicting the last floor is gonna be Purgatory or something akin to that, given the floors already are based on the Nine Circles of Hell (Glacier being the ninth, Tomb being the sixth, and Vestige (Chapter 3) is the third). As for the final boss, it's anyone's guess.

Awestin11, I believe the final boss will either be the third mirror battle (forgot their name) or what you said, with the mirror boss making a last stand.