I could see this working for are Reshiram, Zekrom, and Kyurem. They might be able to go back to their original state of one Pokémon, though this would also force them to fuse, which is something Groudon and Kyogre don't so when they change to their original form. If we ever see them returning to their single form I can see it working through primal reversion or another (new) feature.
Mew is one of the first legendary Pokémon and said to be the ancestor of all Pokémon, so I can imagine it getting a primal form for itself too. I can see the Pokémon Company doing this for the same reason Mewtwo is getting so much attention and two Mega Evolution; nostalgia.
I don't actually expect more primal Pokémon, but I could see it working for the ones I mentioned. Especially if it requires the Red and Blue Orbs I don't think others will be able to do it.