I was looking at the official site just now in the mascots page, and saw this under Primal Groudon (image 4/9): "Spears of rock wrapped in magma stab out at the target! What move is this?!" Under Primal Kyogre (same image number) it says "What move is this?! The pillars of water that have been unleashed are assaulting the target!" That practically confirm that they'll get signature moves that have not been seen before! Finally my precious Weather Trio will have the signature moves they so deserve.
Oh I also found this nice tidbit on the top of Primal Groudon's part "Groudon has long been the heaviest of all known Pokémon. With its Primal Reversion, it smashes that record to pieces!"
For Primal Kyogre it's "When Kyogre undergoes Primal Reversion, it becomes Primal Kyogre, and the length of its body more than doubles!" Damn those things are massive! Actually that may be an understatement. Here's the link to the site if you don't have it:
https://www.pokemonrubysapphire.com/en-us/pokemon/legendary-pokemon/primal-groudon-and-primal-kyogre