While I don't think that this will happen, and certainly not tomorrow, I don't think it's necessarily the most wild shout. McGlynn is obviously at least considering the 3-4-3 and that necessitates either Vaughan or Poplatnik dropping out to allow the additional central defender. Simply put, I think the burgeoning relationship between Vaughan and Poplatnik is the only thing that might stop him going 3-4-3 tomorrow.
You'd probably need Matthews to really act as the anchor, but I could see a world where Poplatnik drops back and is the more progressive of the two in midfield, and continues to link with Vaughan and the two wingers. It's a very attacking set up, though. Although you've got your three at the back, with that midfield set up they're going to be dealing with multiple runners all game.