So it seems like while Primal Reversion works just like mega evolution, it doesn't actually count towards the limit of one mega, meaning you can potentially use BOTH Primal Kyogre and Primal Groudon, and still use a mega evolution on the side. Now the question is, would you want to use Primal Kyogre? The big difference in terms of ability is that Kyogre gains a relatively useless Fire immunity, but the stats are very questionable, with no less than 50 extra points in the useless attack stat. It basically boils down to a 15% boost in special attack and 13% in special defense - and no extra speed. Nowhere near the Choice items it's using so much right now. For Groudon it's really a no-brainer, though. It'll be interesting to see where it all goes, but the fact that you can use Primals at will without taking your mega slot is certainly going to make an impact.
It's also pretty much confirmed that aside from evolutions to old Pokémon (such as Gallade, Dusknoir, etc.), there is indeed no Dex expansion. While people argue that HGSS didn't have one either, not only was the Johto Dex bigger than Hoenn's, but the main game made much less of a percentage of a whole thing than RSE used to. This is why it's so much more of a problem now than it was then.
Finally, the new Hoopa is confirmed to be an alternate form, with the transformation triggered by a key item. So Magician actually makes some sense after all... too bad there's still the major issue of it being a terrible ability even when it works properly.