Zenos is going to be the most important character in the plot of FFXIV for the rest of ShadowBringers, and the rest of the story depending on how SE handles his importance.
Right now, there are many key factors that highlight his role, and why he'll be so prominent.
1. His manufactured Echo, the Resonant.
He has managed to attain a power beyond anything that was thought possible in the story. He truly embodies the Garlean philosophy on not relying on the power of gods, but instead has something that manages to not only equal it, but come from the works of man. This allows him to have influence over Primals as well, which will obviously be very important concerning the true nature of Hydaelyn and Zodiark, most likely taking either one of their powers for himself. Not only that, but it also upped his power significantly as well, where even in his body hopped form, he managed to beat his own self harboured with an Unsundered Ascian, showing his strength with this tool. He is arguably THE strongest villain we have right now, with a position to match to boot. He could most likely single-handedly wipe out the cast barring us right now.
2. He's is Varis's ultimate irony
The man who is ruled by the Ascians, his grandfather being an Unsundered, with him being a puppet king, engineered only for creating calamities. He has no real power, no control, things are out of his hand and he deems the only way to win is to create a race powerful enough to fight them. But he forgot, he already has that man. Zenos, his own son he despised, is the only Garlean not to be manipulated by an Ascian, tempered by a Primal, and is strong enough to overwhelm and force them to flee. Even Gaius, who survived, was a pawn of the Ascians, and though he may managed to defeat the Minors and the Red Masks, he is still too far out of reach for the Unsundered and their machinations.
He's everything Varis wants, yet he hates. He's his creation to fight the Ascians, but he never realised it. Zenos has everything Varis wants, the power and freedom to fight and choose his own destiny, not laid out for him, but he hates the monster he is. This is probably the strongest case of dramatic irony in the whole game, concerning what Varis wants and what he already has.
Zenos is in the perfect position to choose how the story progresses. He is the true Wild Card that can upset the balance of everything. Anything he sets his mind too, he can and will achieve. He foiled both Empire and Ascian plot by sheer-determination to reclaim what he wants, to stop the production of Black Rose by killing his father and beating his own imposter. He wants a fight with us, and he will pursue us by any means. This means if he deems any other obstacle to get in his way, he can very well see the Ascians as being a necessary to take out. If only his father knew what he have achieved.
3. He is truly our perfect antithesis.
We are both two sides of the same coin. If the WoL ever gave into their bloodlust, the thrill of battle and used a might makes right philosophy, we would end up as Zenos, while the opposite can also be true as well. We both possess unimaginable power, strength and resolve but there is a difference. Zenos is merely strong as he is because he was born with it, while we had to climb a mountain to reach the place where we are. But there is also another reflection on that as well. While we earned our power, we are also given power by the very God that sustains our world as well, alongside numerous other benefits as well. All of Zenos power is his own, and what he has been able to make to be on par with that as well.
He is the villain pre-Emet-Selch to have the most interaction with us. In his mind, he has a very warped kinship with us that we may or may not of returned and see in him. No other villain is as personal with us as is Zenos, for striking down our friends, being part of the Empire, having numerous deaths and sacrifices on his hands and representing what we see wrong with the Empire. No other villain apart from Emet-Selch can come as close as this to us. We are truly made for each other, whether we realise this or not.
These are the 3 major points why I see Zenos is the perfect villain for FFXIV and for moving forward.
P.S. May change/update to make points clearer/ add more.