[Spoilers] Why Zenos is the most important character moving forward

[Cilia]

I somehow doubt that Zenos being named successor would mean anything because it's highly probably that Murdering the Emperor is not legal in the Empire, which would preclude any legal transfer of power to the traitor.

This was a direct refutation to Theo's claim that Varis would not have named Zenos as his successor due to his unhinged, warmongering personality making him unfit to rule. The Encyclopedia Eorzea Vol. II contradicts that, stating Zenos was named Varis' successor immediately after he took the throne to avoid a succession crisis like the one he had to deal with. (I need someone with the actual book to corroborate that, as I don't have either book myself. I got to save $ for more important stuff... like Metal Wolf Chaos...)

That said, the only known witnesses to the murder are

1. A disgraced, rogue former Legatus and
2. A filthy savage

... so I doubt the fact Zenos killed his father would get in the way of him claiming the throne if he lied about Varis' cause of death.

Either way, Zenos doesn't care about the throne, so it's a moot point. He has no interest in ruling, only continuing his "hunt," so him being the heir to the Imperial throne is completely inconsequential.