I KNEW Alter Genesis had to involve either Dialga or Arceus. It just made too much sense. I wonder if we'll get a Sword & Shield Pokemon in the set, like Magearna in Steam Siege. Perhaps not, since that set released around the time of the Magearna movie.
As for SM12a speculation:
-With the huge success of GX Ultra Shiny, I'm sure they'll do shinies again.
-We can probably expect a gold Zeraora-GX, since all of the gold cards in GX Ultra Shiny (barring Koko) correlated to Pokemon with unobtainable shinies in the video games (at the time).
-My personal prediction is that we'll see something like Ultra Necrozma & Guzzlord TTGX. Seems like an appropriate "big Tag Team" kind of thing to end the generation on, plus Mitsuhiro Arita hinted at another UB Tag Team in the Unbroken Bonds interview.
-I would predict alternate full arts of Red, Giovanni, and maybe Blue using their (U)SM designs, but the cards aren't staples on the level of Guzma and Cynthia so I'm not too sure on this.