Zenos will return in 7.0

[Absimiliard]

We didn't see him dissolve into aether, so I do still have some small hope we might see his return.

It's possible there was nothing left within him to facilitate dissolution. If Zenos used up the aether of his soul in that fight, then he would be well and truly gone; his body naught but an empty shell. Hydaelyn lasted long enough after completely draining herself to share a few parting words (and a gift for the WoL) with the Scions, so it stands to reason Zenos would be able to hang on long enough for a brief exchange as well.

Admittedly Hydaelyn did dissolve after her death, but her body was also made up entirely of aether. Zenos' body is quite physical, and he was nowhere near anything resembling a lifestream to which he could return even if his soul is still intact.