Just... Just no. This is the first time where the PC is going to be an actual important character (Last of the dragon hunters and whatnot), I don't want it to go more than that. Plus if you are king how do continue your adventure.
I don't know, being the prophecized reincarnation of an ancient hero seemed pretty important to me. Granted, you didn't know you were supposed to be the Nerevarine at the start of Morrowind, but by the end of the game, you would know your role.
So important that Azura politely "de-friends" him and he kicks rocks to Akavir without much consequence.
That's because the Nerevarine had done what he needed to do at that point. I'm sure that the importance of a dragon hunter would diminish considerably once the threat of dragons burning cities has been dealt with too.
But in any case, I doubt the player will become the Emperor, because then Bethesda has to define a canon identity for the player charater, since while you can justify the identity of a hero who fades into obscurity after his role in the grand scheme of things has been dealt with, it's pretty hard to do so for the new ruler of Tamriel, and Bethesda has always left the identity of heroes from past games in the series up to players to decide, so I doubt they'd stop now, and I wouldn't want to be emperor anyway, because I wouldn't want to have a canon identity defined for the hero of Skyrim in future games that is not a character I could see myself playing, and in any case, I'm not even sure I'd play a character who would want to become the emperor. I think it's best to let players enjoy the glory of being the hero in Skyrim, but still allow characters to fade into obscurity in future games.