Something to remember is that Zenos is the PC (Azem)'s foil - their equal and opposite.
Thus while the idea of Zenos being some long-lost reincarnated friend is appealing considering what we know about Azem is nice, it's not one I'm putting much stock in because it would break that paradigm. Zenos is... a newly-born soul, perhaps fresh as can be, as opposed to the PC being the heritor of an Ancient's soul.
The "equal and opposite" even continues into Endwalker: as revealed to people who paid close attention in Shadowbringers' finale, Azem is represented by the Sun, which appears both in an alchemical symbol on their memory stone and as the glyph used to summon aid before going against Elidibus-as-Warrior of Light. Zenos, on the other hand, is pursuing Zodiark and as per 5.5's trailer will be awaiting us in his purported prison: the Moon.
Tarot stuff. Among other things Sun is a card representing optimism and triumph, while Moon represents chaos and madness. Themes.
I'm not exactly for a "Zenos Redemption Arc," but I wouldn't be opposed to one if they did it well. Problem is he's already flat-out stated he has no interest in making amends or changing his ways, fully embracing evil for the sake of his own selfish goals. Based on what we know of him we could - at best - temporarily bend him to our will with the promise of giving him an exhilarating duel to the death later, but even that would be cast aside once all obstacles to his desire are removed. And given his apparent desire is not just to battle to the death (again) but to have us harness Hydaelyn and Zodiark's power, respectively, beforehand...
Finally, as I recall from the presentation unlike Shadowbringers focusing on moral complexity, Endwalker is going to try to focus on sheer epicness. (Epicosity? Either way, already apparent in 5.5 as we release Tiamat of the First Brood from her imprisonment and she curses the Ascians as evil, and nobody bothers to correct her; then we assault the amalj'aa homeland with her at our backs to kill a shoddy knockoff Bahamut.) As such I'm not expecting either Zenos or Danny Boy to have more complicated goals than what they've stated - getting another duel to the death and committing omnicide, respectively. Even if they do get some sympathy from the audience (if not the characters in-story), intentionally or otherwise, I'm not expecting either to be redeemed (or be redeemable at all).
While we are going to the Moon, that was a big moment and thing in IV, and all the Fan Fest goodies are IV toys... I'm not sold on the idea there's going to be lots of IV parallels. Heavensward had lots of IV feelies too, but other than dragoons (OK, a dragoon) being prominent in the story I don't remember any big allusions to that game... and despite the game having more references and shout-outs than you can shake a stick at, the MSQ at least does maintain a large degree of originality.