Honestly, I have a feeling the "new Pokemon reveal" is going to be Zeraora. It's the only Gen VII Pokemon that hasn't gotten an official reveal yet, despite the fact it's lurking in the data files and thus a lot of us already know about it. Along with this, mythicals tend to get featured in movies, and considering this is largely a M21 announcement, this is definitely lending to Zeraora being the so-called reveal.
As a result of the timing, this is going to be very similar to what happened with Marshadow last year, and if they follow the same schedule with him, Zeraora will get released this December. This'll also help plug the missing hole in the legendary event schedule.