I've also thought before about the ideia of Kyogre and Groudon having two new abilities that work just like Drizzle and Drought the way they worked before (infinite weather)... I don't get this Ancient Devolution thing though, what are they technically supposed to be? forms, megas, new mechanic?
Mega Sceptile and Swampert were obvious and Sceptile getting the Dragon-type was expected. Mega Swampert looks funny (how can it stand on two legs with that bulky upper body?) but Mega Sceptile, while it looks cool, I think it doesn't change much from regular Sceptile... I need a better image of its front to see other details...
Mega Diancie was unexpected but if you think about it, it makes sense... I wondered a few months ago that Diancie could have something to do with Mega Evolution as it will appear in the new movie, where we already know a few Megas are supposed to appear, and Mega Evolution is achieved using Mega Stones. Diancie is part Rock-type... Maybe it's unrelated but anyway, it's around this time of the year that they reveal the last secret (meaning a new forme or something similar) about the movies and just like Keldeo Resolute in M15, this is their way of giving more hype to the star of the movie, as I'm pretty sure they will show Mega Diancie in the movie - specially when the movie summary says Diancie still isn't strong enough to create that big diamond the carbink need, Mega Evolution will probably give it that power.
I'm even starting to think, maybe Hoopa will be treated like Meloetta last gen, and get only a short film and the next movie will either feature the Mega Lati@s or they will skip right to Volcanion to be featured in the next movie along with Zygarde, should Pokémon Z (or wtv its name will be) be released next year.