The official Japanese Pokémon website seems to have revealed a piece of new information regarding the Mythical Pokemon Magearna. If one looks into the source code of the website, there are various sections for the different types of Pokemon, from Guardian Deities to Legendaries, but one that stands out is Megas. It just so happens that Magearna is listed under that section, even at the time this article is being written!
Perhaps this listing was not a mistake and we could actually be seeing Mega Evolution return in Pokémon Sun and Moon? Would Magearna be one of the first to gain such transformation in the Alola region? It sure seems likely, but we will have to wait for an official confirmation to be sure. Updates will come as more is announced.