After looking at the calendar, I think you have a fair chance of being right. It seems too big to just be an anniversary, especially since it was set for the end of the month and not the actual anniversary.
It seems more like a prepack update. What pack, though... I am unsure. I was in the gamepack camp until the anniversary stuff. If I remember correctly, the largest updates tend to come with expansions. I will not commit myself to it by any means, but until I have a reason to believe otherwise an expansion is not impossible. Though it is just as possible that it is a standalone update.
I feel fairly conservative about that announcement timeframe, though. I suspect they plan to sit on it for a while after the teasing.
