Loose ends
There's a number of outstanding plot points, which remain "hanging":
- How the Resonance came to be and what its real purpose is
- Whether the Ascian Overlord in Solus's guise was possessing him before he sired Varis's father
- What Solus has been doing all this time
- Why Elidibus was so eager to test the vessel
Unfortunately, Banri Oda couldn't be drawn into answering questions about these quite yet, so I'll be indulging in quite a bit of speculation.
A slumbering Overlord's true colours...
Solus's role in all this is interesting to me. It's odd that an Ascian Overlord should even need to "rest" - they're tireless immortal beings, don't seem to take long to recover from whatever battles they get into and have a singular devotion to resurrecting Zodiark. Even assuming he managed to rejoin a world, why would he suddenly go to rest whilst the other Overlords were about their work on the Source? We know that they don't necessarily tell each other what they're doing at any given point and enjoy considerable autonomy, beyond not messing things up by causing floods that write off entire worlds for the Rejoining. It's a hard pill to swallow that he's just been lying in that coffin all this time. So what has he actually been up to?
Let's assume for a moment that he is possessing Varis (or even a cloned version of him, given his remarks about Allagan cloning techniques), and has been since the tent scene in the Ghimlyt Dark... going over his lines, although he clearly respects Elidibus, to a degree, the implication that he's a "worrier" might be a sign of frustration at him and his methods. He seemed none too pleased about being dragged back into this all, and highly critical of Lahabrea, but again, dragged in from what? As an Ascian, what else would he be doing with his time? Regardless, the lines at the end of 4.4 essentially involve him instructing Varis to do his duty, and reminding him he has no decision-making authority in the whole affair.
Following that, there is the scene in 4.5 regarding Seiryu's Wall - Varis is hesitant about resuming the production of the Black Rose but wishes to do so to preserve Garlean lives; Solus, meanwhile, is excited about the possibilities it has in a place where the deluge of Light has worked on the local aether, suggesting this will enhance whatever effects it has if, for instance, the flood were farther ahead than on the Source, e.g. on the First, which is no doubt where he will be headed. Then comes the speech in the tent scene; for the first half, Varis mostly deflects the Alliance leaders' barbs, and the portrayal is consistent with his usual self. Then come the flickering lights in the second half, with the black moths drawn to them, which is a classical signifier of possession. The speech he gives is at odds with wanting to preserve Garlemald, because he admits that his new plan would involve Garlemald's erasure, and he speaks of taking things back to how they were at the beginning, before the races of man split... (bearing in mind Solus had also alluded earlier to restoring mankind to its rightful state in his earlier chat with Varis.) This is all with the aim of eventually hoping to render the Ascians a non-threat... but essentially by delivering exactly the fate they intend to bring about, anyway. The dramatic flare was also at odds with Varis, who but a few cutscenes before, Solus had described as being unable to mask his feelings well.
Now, if you re-cast this as Solus possessing Varis, it has some interesting implications. One, he clearly likes to parrot some of his own lines through Varis. Two, perhaps that mindset is not at all Varis's - pretend or otherwise - but Solus's. Better yet, perhaps he is frustrated at the progress the Ascians are making on the Source and Elidibus's fairly (to him, overly?) cautious leadership, and has a masterplan to take Elidibus out of his role and assume the part of Emissary for himself. A more radical agenda, yet, could be to try take over the very role of the Crystal of Darkness and unshackle himself from servitude to it, but it's not clear to me that he's that ambitious or whether it's even feasible, although we know there's means by which such a crystal could be created anew. Three, when he speaks of this becoming a family enterprise, perhaps he is not just joking and means to use Varis to come and go as he pleases within the Empire. I think edging Elidibus out of the scene seems to be a pretty reasonable possibility as to where Solus could be taking this. In either event, he needs the Rejoining to proceed apace, and tilting the First more towards the Dark through indiscriminate slaughter via the Black Rose could be his preferred means to do so.
Like (great grand)father, like (great grand)son...
In turn, that brings Zenos back into the picture. I've been rather dismissive of the prospect of whether being sired by an Ascian would have any effect on the resulting offspring, and this all ties into an as yet unknown chronology of events, but reflecting on that, it's not clear to me why it must be so that the offspring would not be affected in some way. Life on Hydaelyn is both corporeal
and aethereal in nature. Ascian possession appears to erase the host for the most part and forms a bond with the body whilst the Ascian is possessing them. So what if this has in fact altered the Galvus bloodline? It may not express itself in Varis or his father, because they have the regular Garlean inability to manipulate aether, but Zenos's potential has been unlocked through the Resonance. It's not clear to me whether an Ascian - Solus included - guided this project, but it most likely took advantage of Allagan aetherochemistry to manipulate the Pureblood genome so as to remove any inhibitors it has to utilising magic.
That could also lead into what is so special about him as a host - if the above is true, he'd be part Overlord by nature, with the Resonance unlocking that power and giving him hold of something like the very potent Echo an Ascian Overlord would possess; he'd be a synthetically produced Overlord/Pureblood hybrid. So, rather than Aulus just implanting a particularly powerful imitation of the Echo in Zenos (Fordola's version is nothing like Zenos's, for instance), he could instead have been harnessing his latent potential. This would make him an excellent counterpart to the WoL.
Assuming these two plot elements both hold true, I wonder whether they could interact, and we could see Solus edging out Elidibus (I can't help but wonder if they've decided to move some of his intended role to Solus, due to Solus's popularity - Elidibus's has waned a lot since he faded into relative obscurity following ARR), assisting Zenos in reclaiming his body and teaming up with him to sow as much chaos as feasible on the Source and First. Zenos would serve as a useful means to Solus to distract you - he wouldn't even need to kill you whilst he busied about his work, but just unleash this obsessive frenemy of yours onto you, in order to distract and slow you down. I don't see Zenos taking out Varis as a good thing for the Empire (as a nation, not Ascian tool of chaos), as Varis is at least mentally stable and interested in the nation's and its people's continued existence, whereas Zenos essentially oppressed two regions with crushing brutality, having no regard for any life... on a whim, and allegedly has had no change of heart or desire for penitence. However, that's assuming Varis will be around to dispose of, and is not possessed by Solus. If Zenos were to cause such chaos, given Solus's own agenda and style, he might even embrace it.
I also like the comedic value of Solus taking some joy in repeating Varis's usual lines and severe tone.
Granted, they could just do whatever feels "cool" in the moment (rule of cool, and all that), and Varis may not be possessed and simply managing the situation as best he can, but I think this would make for a pretty awesome plotline/twist. I don't consider it plausible that Varis will unleash the Black Rose on the Empire, nor do I think he relishes in using it on the Alliance - his words pointed to a strong reluctance and that it was being done out of perceived necessity to limit Garlean casualties. The alternative is he may plan to use it on the Ascians, but that'll be futile since they'll just jump bodies without the right tools at hand. In turn, if it is Solus, he could just want to ensure Elidibus doesn't get in his way, but if he really thinks he's better at the job than Elidibus would be, I could see a takeover bid regardless.
If all they intend for Zenos is to be a wrecking ball in the Empire, I'd say meh. Waste of potential uses they could put him to.
Even more dangling loose ends...
On a side note, I also wonder what they're planning to do with Construct 7. Its ability to open portals to the very centre of the planet was interesting - the implication being that this could access the Aethereal Sea? - but I have a feeling that the way this raid content could be used is enabling Garlean reinforcements to circumvent Seiryu's Wall, increasing the pressure on Dalmasca and reinforcing Noah von Gabranth's IVth Legion. Yoshi P had alluded to Orbonne tying into some aspect of understanding 4.5, but as yet, there's nothing any of it clearly tied to that I could see. Again, it does reinforce the possibility suggested by many here that there was some re-writing that went on as the patch progressed, and we all know they were severely pressed for time, so mayhap this will resurface at a later point, tying in either to the two Crystals, or Dalmasca... or maybe something else.