I knew the Rock/Zygarde from the opening was going to be the megalith from the moment the previews/summaries came out. Been saying it for a month or so now but no one believed me. The reason that Steven returns is probably because of the megalith. As it was, Steven has always been suspicious of Lysander since he took the megalith. Remember that it was locked away for a reason, so I am assuming that it has the ability to have a mind of its own, which is probably why all 3 of the legendary Hoenn region Pokemon were awakened when the Megalith came out. Anyways, I assume that Serena and Maron will fail in getting Chespie out as the megalith becomes activated. I'm assuming that the megalith companies with the Zygarde energy or cell that Squishy left behind in Chespie when Chespie helped it escaped. Anyways, this cell or energy probably has a lot of hate towards humanity due to the way it was treated and thus bent on destroying the world. How will it destroy the world? By going to Anistar city and taking all the energy from the Sundial that is there. Remember, that the sundial is considered a primary location for Mega Energy. Also, you can see from the opening that it is crawling towards Anistar and the sundial since the glowing rock is the sundial. Anyways, it will be up to Steven, Serena, Clemont, Myer, Team Rocket and probably the gym leaders to hold off the megalith while Ash and Alain go in and get Chespie out. They obviously succeed and that is when Squishy and Z2 combine into their perfect form and defeat the megalith.