I had this brief, vague idea cross my mind one day of an extension of Skyrim's plot. There are a few other theories about potential loose ends that we could see added to, and there is also a new, official DLC coming up soon(-ish), but here's what I think:
The Dragonborn has destroyed Alduin. He or she has dealt with the Dark Brotherhood, whether that means becoming its Listener or destroying it. Though a Civil War may or may not continue to rage onwards in a cloud of turmoil, Skyrim has found a brief moment of relative peace. This is not quite the case everywhere, however.
In Cyrodiil, much political chaos has torn the province apart. If Emperor Titus Mede II is alive, he is said to be incompetent and of plenty ill-intent. If dead, others seek his position. In any case, the throne shall soon see a new Emperor.
A courier approaches the Dragonborn, giving a note sent by Delphine. The Dragonborn meets her at Sky Haven Temple, and she explains that dragons have begun to disperse erratically, their leadership disrupted so violently. They have crossed borders, and attacks in Cyrodiil have been suffered several times reportedly without any preparation. Dozens have died, burnt in their own homes, crushed under a dragon's footing, or impaled and mutilated in their jaws. The Dragonborn is sent to give aid to a town there (sorry, I haven't played Oblivion, so I don't think I'm exactly qualified to be mentioning any names of locations or people; I simply won't). The gate to Cyrodiil is now opened for the Dragonborn to enter its Southern neighbour. He or she travels to the aforementioned town to find a man, perchance in his early-thirties, fighting off a dragon. Slaying it (accompanied by a finishing move animation) he is seen absorbing its soul. He is Dragonborn.
When approached, the man is speechless. The Dragonborn is no different ("..." is the only conversation option). He eventually introduces himself as Difiscus (for the purposes of this summary, though any name is fine), and the player tells him that he is Dragonborn. "You mean like the Emperor?" he would say, at which the Dragonborn would correct him. Difiscus explains that his grandfather was killed by the attack, and his house was destroyed by this monstrosity of a dragon, now reduced to ashen bones. He goes on to tell the Dragonborn that his wife and daughter were taken by the Thalmor several months ago. The Dragonborn then tells Difiscus that he should visit the Greybeards, and thus the two depart into their separate paths. Several days after successfully prtotecting the town until they've erected defences, the Dragonborn receives a summoning call from the Greybeards.
The Dragonborn arrives at High Hrothgar and is greeted by Arngeir and Difiscus. It is explained that Difiscus has been taught all that can be taught, and any further training would require the consuming of Dragon souls. The Dragonborn may then choose to personally see to it that Difiscus has his share of dragon-slaying, or give him up to the Blades to train and fight however is necessary for the protection of Skyrim.
With or without Difiscus' aid, the Dragonborn continues his/her campaign in Cyrodiil, and the Thalmor are assuming control of defensive measures - or so they say. The Thalmor seem to quite evidently be establishing a political foothold in Cyrodiil. They are beginning to gain some respect and create a military presence. Imperial influence and morale is of a dwindling minority at this point. Ultimately, the Second Aldmeri Dominion attempts to inveigle the Imperials into trusting them and giving them power. This occurs insofar as to the passing of an agreement for the Thalmor to virtually unconditionally occupy Cyrodiil whilst the dragon crisis continues. And indeed it does. When dragons enter the Imperial City, the chosen new Emperor is killed, and any forseeable glimmer of Imperial hope for their empire is felled. The Thalmor see this as an opportunity to fully and legally take over not only Cyrodiil, but their entire empire. But unbeknownst to all is that another candidate exists.
Difiscus now wishes to claim a right to the throne. Being Dragonborn, he has a legitimate, justified reason to take the title of Emperor as his and not a trinket of the Thalmor's. His growing resentment for the Elves brings him to the Dragonborn, whom he asks to help him in his ascent to power. As Difiscus stations himself in the Imperial City (or elsewhere - it doesn't matter), the Dragonborn infiltrates Thalmor outposts and starts looking for names. He or she collects several documents and brings them to Difiscus. They uncover a name: the name of the Thalmor's proposed soon-to-be Emperor. Ozerri (again, this is a placeholder and can be replaced with any name). Thus begins the hunt.
Difiscus then consorts with a myriad of people, secretly spreading the word of who this Thalmor leader is. Meanwhile, the Dragonborn is tasked with locating Ozerri. This brings him or her to the Thalmor headquarters in Cyrodiil, upon which Difiscus' wife and daughter are stumbled. Imprisoned in a locked room, the Dragonborn can do nothing but tend to the wife's pleas of ensuring that Difiscus is safe. After having completed this quest, the Dragonborn returns to Difiscus and tells him that his family lives. It is at this point that he feels the time is right to announce his right to the throne. He publicly challenges Ozerri's political standing and claims to be descended from the blood of a Septim. This is received with much approval by the locals, but with suspicion from the Thalmor. They try to abduct Difiscus on the spot, but the Imperial witnesses retaliate. Difiscus absconds, retreating to a hidden shrine to Talos outside city walls where he hides.
Several days pass, after which the Dragonborn receives a letter from Difiscus to come immediately. The Dragonborn enters his damp, abandoned, dark abditory and he informs the Dragonborn that a local (the locals know where he is hiding because they hid him) was told to give him a letter. It said to meet at the Imperial City Arena for a final confrontation. Difiscus is expected to arrive alone, so the Dragonborn is tasked with making sure the battle goes smoothly. He or she proceeds to kill off several Thalmor archers and disable traps before the battle begins. However, once the Dragonborn ensures that the battle will be clean and seats himself or herself amidst the crowd, the gates open, but Ozerri does not emerge: rather, he appears standing on a balcony. And instead, much to Difiscus' horror, his wife is dragged into the Arena. They speak audibly, having not seen one another in so long. But then a flame; she is set alight as her Thalmor captor drops his torch to the ground and begins to laugh. She collapses in a pool of her own bloodied tears. Ozerri also begins to cackle, and he flounces out of view. At this, Difiscus is enraged and Shouts, Unrelenting Force launching the Thalmor in front of him several metres away, and also breaking the gates on Ozerri's side open. A Thalmor voice can be heard calling for archers, the crowd goes rampant, and above it all, the Dragonborn receives a new quest objective to use Call Dragon.
The Dragonborns chase Ozerri through the streets of the Imperial City, pursuing him into the White-Gold Tower. Defeating all of Ozerri's personal guards, they encounter him finally in the throne room. A battle entails, from which Ozerri is killed, and Difiscus takes his seat at the throne, ergo, the conclusion of the Cyrodiil questline.
So, what did you think, if you managed to read through it all? Did I misinterpret anything? Do you like it? Are there any facts or assumptions I got wrong? Any feedback would be appreciated, critical or praising. I made up most of it as I wrote, so I could have easily created some plot holes. In any case, though, thanks for your time!