My guess is that the Ardbert lookalike is Elidibus, and this is on the basis of his dialogue and what Unukalhai says about him. The character's comments, that it is his destiny to see "Our dreams" realised reminds me a lot of the dialogue by Hades during the trial. Then, there is the fact that Elidibus, at the end of 5.0, commented that it is the fate of Warriors of Darkness to be destined as those before them, to meet death at the hands of Warriors of Light... and finally, we have Unukalhai's dialogue about him.
Chapter 3: Sophia:Upon our first meeting, my master spoke thus: I am not the Light's servant, but for the present I must serve. One cannot arbitrate equilibrium if one is not fluid of stance.
Unukalhai: 'Tis well that another world should not share the fate of mine own, but you must be careful─the Ascians will not take well to your intervention.
Unukalhai: Of those who yet remain among their number, Elidibus in particular concerns me greatly. It has ever been his aim to seek equilibrium, even when the means to do so is seemingly at odds with his brethren. I can but wonder to what lengths he might go to account for your actions.
This, to me, might all be implying that Elidibus is taking on the guise of a WoL - he won't need the body, as he can shape a host however he so wishes - to achieve his agenda on the First. This is a lot more promising than some idiotic scheme to summon the self-same Shard WoLs that have proven incompetent at defeating you in the past. Even though Hades fell, Elidibus is one of the few lifeforms who has a genuine chance to fell you. Shard WoLs will be too weak. So my guess is he will avoid direct confrontation for the time being and try ensure the First is put back on the brink of destruction, assured that the chaos on the Source will suffice to ready it for a Calamity.
The other entry is a mystery but my money is on some manner of Ascian or a Warrior of Darkness, like Cyella. Whoever it was must have witnessed the dialogue between Varis, Elidibus and Zenos, because it is at this point that Zenos formulated this new agenda. I've heard quite a few suggestions made, including Unukalhai, but I don't think he makes much sense, because he's still in the Rising Stones, still in a child-like body (this was a full grown Midlander man; it's only "small" next to a giant like Zenos), and has dialogue being updated. It'd require far too much for it to be him, so Occam's Razor would suggest otherwise. My primary guess is an Ascian seeking to make use of Zenos's new found agenda, or a Warrior of Darkness with an appetite for some chaos. I had thought it might be a soul connected in some way to a neutral ancient during the whole Hydaelyn/Zodiark summoning fiasco, but again it requires a lot more legwork to explain than the prior two possibilities.
I don't think this person is a Hydaelyn devotee, either. Leading Zenos to Zodiark will put her at significant risk. Even if this resulted in Zodiark's ultimate destruction, it could just as easily result in hers.
Some have speculated Lahabrea, since his destruction was assumed and never assured, but surely Elidibus would sense this? Unless the Eyes somehow stripped his darkness aspecting/otherwise changed the hue of his soul and resulted in Elidibus being unable to identify it... still, I am sceptical that this is who it is. I'd like to see Lahabrea's ascendant form and always wanted more of him, but I'd also rather they don't keep recycling characters, so kind of on the fence about it.
I get the impression that Zenos doesn't really seem to have much knowledge about how to implement this plan, so I guess it's no surprise he decided not to even use the Empire towards that singular end. This new figure will provide him with that know-how, however this might end for Zenos. I could see Zenos succeeding in hijacking Zodiark if he was summoned with a small amount of aether initially and then gorged himself on more and more to empower him. It may yet be a foolhardy plan that ends with his demise and frees Zodiark. Or others may be right, and it may be some trick he's discovered which does not require him to overpower the Primal, per se... that has scarier implications, as Hydaelyn would be susceptible to much the same and not just from Zenos, but anyone who discovered this ability.
As to the Pureblood being used in that Arch Ultima device, who knows? To me, it appears Varis may have objected to such projects in the past, but with the Rejoining as a new agenda and goal for the Empire, as perceived by him as necessary to re-unite the world and drive back the Ascian threat (assuming he was being genuine in the tent scene speech), maybe he thought why waste all the research Castrum Abania yielded? Especially if this was some Populares prisoner of war. Perhaps Elidibus was emphatic on making use of this research to further the Garlean war machine and left little choice in it. After all, he was convinced to use the Black Rose to limit Garlean casualties. I can only hope we get more insight into Varis's state of mind later on. I liked him as a character and consider his death to have been too early, but with confirmation that Zenos will not even use the Empire in his pursuit of Zodiark, a civil war looms. This, I think, given Krile's comments about the risk a civil war poses to the provinces (and Garlemald itself, I'll add, as most Garleans are oblivious to what's really going on behind the scenes and are hostage to grand schemes), gives me some hope that we'll eventually venture into Ilsabard, to diffuse the situation. So I'm glad, in a way, that Zenos has fled his throne. I'm intrigued as to whether this "Lord Nerva" will play a big part in the future or if it's just a name being tossed in. As much as I enjoy the Ascians and Zenos, I think it'd be nice to see some more grounded politics in the Empire come back to the forefront for a future expansion, and perhaps this will be Gaius's destiny in the story - to restore order to his homeland. We also have this mysterious device missing from the capital - I wonder what it is. Are they finally going to put Construct 7 to use? Would Gaius know about it, e.g. via reports he's receiving from friendly eyes on the ground? Is it something else? Black Rose related, even though its production facilities are gone? Eager to see.
I enjoyed the Estinien/Gaius RP scenes, but as usual, I wish they'd elaborate a bit more on what triggered the overt aspecting to Darkness in Zenos, as well as his newfound ability to teleport like an Ascian. I guess this will be revealed later.