Possible hint at where Labyrinthos is located.

[DaemonInazuma]

What if...it's backwards? Zenos is actually going to fuse with Hydaelyn, and wants us to fuse with Zodiark. Zodiark was such a powerful opponent that Hydaelyn could only cleave him into parts, making him a dangerous foe that Zenos would want to conquer, and using Hyadelyn would prevent any tricks such as just splitting Zodiark to make him weaker, something he'd be aware we could use against him if we used Hydaelyn. Zenos would have full control over that power, forcing it out of play and could essentially force a fight on the grandest stage of all, with the WoL using a god of immeasurable power against Zenos, who is bringing out Hydaelyn's true power in another form. It would also play as a form of divine irony, we having to use Zodiark, the thing we've been preventing from awakening, against Hydaelyn, the thing we've been actively helping.

I like this theory as well. The only issue I have with it is our character and our allegiance to Hydaelyn. They called this into question in 5.5 with Y'shtola and Krile asking when we heard her voice last. I feel being that we are a reincarnated Azem, we won't align with either Zodiark or Hydaelyn in the fight and we will tread our own path instead a la calling on our reflections in other worlds to join us and becoming more powerful in that way rather than subject ourselves to the ancient primals.

Overall, Zenos and Fandaniel's plan seems to be aligned with releasing the ancient primals by forcing our hand to do it.

I think that Zenos and Fandaniel are definitely going to have it out at one point as well. Fandaniel has made his intentions clear in 5.4 that he wants to die (like truly die as ascians are reincarnated until their purpose is fulfilled) and the only way to accomplish that is to start the final days and release Zodiark and Hydaelyn, ending everything. This goes against Zenos who's intention is to fight us on the grand stage in a final form per say. He would rather keep this fight in perpetuity rather than destroying everything.

I also think the secret Fandaniel holds is what Zenos truly is. He says this at the end of 5.2 by saying Emet-Selch was able to accomplish something relating to Zenos's dreams of the final days since he was a child. My theory is that Emet was able to implant either a part of his soul or an ancient's power into a sundered being's soul. That is what makes Zenos so powerful. Similar to us absorbing Arbert and becoming powerful enough to take down Emet. Zenos and the WOL are basically similar in how they became so strong compared to everyone else. That secret will become very important and give us insight into who Zenos really is.