According to legend, only those with the Dragon's blood, the Dragonborn, can rightfully take the Ruby Throne. This is why Talos was destined to rule and unite all Tamriel, and is also basically the whole exposition to why Molag Bal is such a threat in Elder Scrolls Online. If this is the case, and Miraak had successfully returned to Nirn, would he have had a rightful claim to the Ruby Throne?
It's an honest question. I'm not refering to the legality of it by the Empire's standards. I doubt Titus Mede would just stand aside and let Miraak take the throne. But consider that the only time that the Empire actually grew in strength was when it was ruled by the Septim bloodline. It has had rulers both before and after, but never any quite as successful. Perhaps only the Dragonborn are "meant" to rule Tamriel as a whole.
If this is true, then my theory is does Miraak have the "divine" right to rule in the same way that Talos did and, more or less, the Last Dragonborn.