I think for an expansion of Skyrim, that there could be another Dragonborn. A PC that can also learn dragon shouts easily and absorb dragon souls. But what this individual does is very different. This other Dragonborn restarts/takes of the Dragoncult. As such it enlists dragons, undead Draugr, and living PCs. He/she could make new Dragon Priests, conduct raids of places, like High Horathgar, which the player must stop. In early confrontations with the other Dragonborn, Dragon souls might be taken from the player, so that they lose shouts, but also he gives hints where to learn, or where to create, new shouts. THe player might have to hunt specific dragons helping the Dragon cult, kill new Dragon Priests, deal with larger numbers of Draugr outside of "dungeons." etc. This expansion could introduce new shouts; new uses for dragon souls, like the creation of masks; could have new enemy types, like undead Dragons that are Shadow, Spectral, Corporal, and Skeletal; or ones that breath lightning. There are large possibilities, along with a final confrontation with the other Dragonborn, which might allow the player to destroy the Dragon cult, or take it over. This would be along the lines of the Knights of the Nine expansion for Oblivion, in that there wouldn't be a "new" place for the player to go, but to involve more of the current world in a long, interesting, and fun, series of quests.

Not sure about the story exactly, but we do need some more factions to join/fight, and the Dragon Cult sounds awesome.

I'd be so happy if they added an Arena, like in Oblivion, just not as reptitive.

http://www.gamesas.com/topic/1336807-unofficial-dlc-discussion-spoilers-thread-3/ for possible DLC/expansions for Skyrim. Please use it.