It may just be wishful thinking on all of our parts, but I'm having a sneaking susipicion that Gamefreak didn't abandon all the plot threads left hanging in X/Y and ORAS. Magearna's core has both XY and Sun/Moon colors (red vs. blue) and the same goes for Zygarde-100% too.I never noticed this before, but Magearna's core has the XY colors. It's odd, Magearna feels like a gen 6 Pokemon and Volcanion seems like it's a natural fit in Alola.
Additonally, Volcanion is an undersea volcano pokemon (at least I think so...) but it's a Gen 6 'mon while Magearna is Gen 7.