Dragonborn being dead means so is the plot...
But! That's a much better idea, Roze. What if we just say the Dragonborn settled down, quit all the 'jobs' that came their way throughout the story. So there's new guild leaders, college arch-mage, thanes and all that, but the Dragonborn is still 'related' to those factions as they've worked with them a long time.

As for the Dragonborn's obvious overpower... They're not impossible to kill, and I like to think I'm pretty good at thinking up antagonists. The idea is this extremely skilled and capable person has all the strength and influence to rule Tamriel, so they don't, like.. get shiv'd and keel over like other Emperors that have existed. The power's meant to be their assurance of survival and their way of rightfully claiming the throne.