I was recently thinking about how to become Emperor, and I came up with the perfect scheme (hopefully).
It would easier as an elf because of the long life-span they have but it's not necessary.
The first step in my grand scheme would be to complete the civil war quests for the empire, making sure to keep the Jagged Crown by reverse-pickpocketing onto Hadvar. Now that Skyrim has been reunited, become Thane in all the Holds, specifically Solitude. After helping Elisif she makes the remarks which normally indicate that she is available for marriage, however this is not the case. So let's assume that you can marry Elisif, and the Dragonborn does so. Here's where the extended life-span comes in handy because you want to outlive Elisif (alternatively you could assassinate her). With Elisif dead, Skyrim no longer has a High King/High Queen, and this is when you declare yourself High King using the Jagged Crown, your title of Thane and the right of Dragonborn to legitimize your claim. I'm also not entirely sure how Skyrim's succession laws work, can only Nords be the High King? Anyway as High King you now need to assassinate the Emperor, by completing the Dark Brotherhood questline. As the High King you already have a position on the Elder Council and you can use this along with your status as the only known Dragonborn and the assistance of the Blades, to become Emperor (assuming Titus Mede II has no heirs). If your claim is not recognized then march on the already weakened Imperial City and become Emperor by force.
So that's my plan to become Emperor, what do you guys think?