At this point, i would honestly welcome any new monster, including the bread monster that we have seen in the videos as a halloween boss to appear instead of this silence from Valve part. They really need to invest more into halloween next year themselves.
One thing i suggested to them for this year was that they would create 1-2 monsters (lets say moving on the ground or air) with instructions to use so that mapmakers could implement those into their maps. Those would be then revelealed when halloween hits and players would be surprised, including the mapmakers. But that idea never got any echo.
In any case, if community can cook up a monster that is animated, works perfectly, i'm sure Valve could implement it to work within the game, assuming it flies or runs on the ground itself or whatever it might do.