Brendan's "hat" is really quite the revelation for me!
Anyway, concering Primal Reversion, I think it's almost safe to say that Regigigas will get a Primal form. From what we know so far about Primal Devolution, the Pokemon goes back to a more powerful state that it had in it's prime long ago, but was lost in the future to the extremely long rest. Regigigas fits the bill to get one perfectly, because it too was found in a weaker state after eons of slumber, and it was extremely powerful in its primal state. The home of its trio is in Hoenn, so I'm sure it would fit into Hoenn quite nicely.
Regigigas was said to be so powerful, it could move entire continents. That titan sure must've exerted some Pure Power Sheer Force in those days! It also built 3 living creatures out of the elements, perhaps using lava or fire to help create Registeel. Regigigas must have been quite Protean, being exposed to and using all the harsh elements in its day. Quite a creature of great Adaptability, wouldn't you say? According to what I remember from in-game lore, Regigigas was sealed away because the ancients feared it's power. It must've ruled with an Iron Fist back in the day! Okay I'm done