Now that the Soul Cairn has been introduced and explored, I think there should be a "Reaper"-type DLC. Players start the DLC by overhearing store-owners (inn-keepers, blacksmiths, pawnbrokers, etc.) speaking about a strange light they saw in the sky the night before. They explain it as if it were a hero's soul reaching Sovngarde. The person then mentions the location it looked like it was rising from and a quest appears on your screen reading "Soul Searcher" (or something along those lines). Being the curious adventurer you have always been, you set off to reach this spot (I don't really have any ideas for where it should be, maybe around Dawnstar). When you arrive, you find a crater in the ground with what seems like a half-dead mangled man, named Gilor (or not) lying on the ground in it's place. Speaking to the man completes the quest and starts a new one, where he tells you that a man dressed in dark clothing and a covered face told him "his time was up" and he needed to "take his place in the Cairn". Scared and confused the man explains that he is a master in destruction magic and he tells you he killed the man, but the body just shot up into the sky and dissapeared. The next quest begins, called "The Sand Man?" (again, that is just a rough idea) and you take off to ask around Skyrim for any information about other sightings or stories of such. Until some random person tells you to find a book that has a story that sounds really familiar to what you are speaking of. So you do. The book is titled "The Keeper and his Keeper" (or something along those lines, of course). Reading through it you gain knowledge of the man that Gilor mentioned. You learn that the man is called a "Reaper", a "Keeper", or something like that and he keeps souls in the Cairn and collects souls to be kept there. The book gives you the location on your map to a temple/sanctuary where you may find a way of contacting such a being. Travelling there will spark bandits, spiders, skeletons, you name it to attack you, but prevailing as usual. Once you make it deep inside and you find a scroll or tablet used to summon this being, you do so. He arrives and he looks weak and ready to lay down and die, but before he does he curses you with the abilities and starts chanting in a forgotten tongue. You then appear in the presence of the Ideal Masters with a quest labeled "The Keeper's Keepers". They then explain what you are and what you need to do, which can be up to your imagination. After a point of doing endless quests and taking countless souls, you decide the luxury has worn out and you want out. You discover that to get out, you must collect a bunch of items and return them to the Ideal Masters' chamber in the form of an elixir or poison. You then have to empty it into/on them to weaken them and make your escape. The only catch is, you have to give the curse to another accomplished soul. In my opinion, a cool thing you could do with this is give the option between one of the Jarls or maybe even Paarthurnax. You are set free, but you are allowed to keep the abilities, as well as the skill tree that comes with. This seems to me to be a nice way to wrap up an official Soul Cairn story line. [Ideas contributed by Andrew Britton and myself]

Another idea is to have a full on war between Vampires and Werewolves where the Circle's Werewolves and the Volkihar Vampires start vigoriously recruiting for the upcoming territorial battle for Skyrim. You are caught in the middle (unless you have already chosen a side) and must decide whether to become a Vampire Lord, a Werewolf Commander, or a Human Legate for the Imperial/StormCloak army (depending on which storyline you followed). Or the Imperials, Stormcloaks, Thalmor, and even the Greybeards band together to keep Skyrim human. Being a human, however, means you must fight both sides and will most likely end up failing in the end. I haven't thought too much on this idea, but it could be a really cool ongoing quest line. Even more you could do with it is make it so along the human path you die and go to Sovngarde to assemble help from the Old, in order to stop the incoming plague. [Ideas contributed by Andrew Britton and myself]
Thanks for listening to my ideas and please give me feedback on them.
I'll keep this updated when I come up with more ideas for each, or if I have a whole new idea.