I will mention 2 other points in support of him returning...

Even though it was back in Stormblood, his mention of binding with a primal is of great note. Until Endwalker, when we basically realize that we have been serving a primal all along, we saw primals as a threat to the world and sought to eliminate them. Now we are using them to help us, and his knowledge of binding them for our bidding, I believe could come in handy down the road.

Also his fabricated Echo has not been put to full use. We've got a couple of soldiers running around with it, as well as Zenos himself, but being the master-mind behind it all, it would be a shame for him to die off before it comes to full fruition.