You underestimate him. A fatal mistake.
He snatched victory from our heroes in Laqua and when they fought the 5 minions the last time, so he's BOUND to have something up his sleeve...<_<
Like I said on another thread (and I think earlier here), I can
totally see him whipping out a Master Ball to unleash a Mega-Evolable Legendary/Mythical (if just to justify the kids needing Mega-Rayqauza and/or Zygarde). Heck, maybe even multiple ones for all we know. He seems the type. And yes, I DO think he'd release his Pokemon besides Umbreon to do it. Heck, for all we know (again) he released Umbreon and it's sticking by him because it WANTS to. <_<
This is all hypothetical of course.
Though on the other hand, they COULD have the PokeGuardians vanquish the Explorers and go on everyday adventures/misadventures like how Ash-tachii still did stuff after dealing with Aqua-Magma, Galactic, Plasma, and Marco Cosmos...

(Maybe a tournament arc to promote Champions???)