After replaying both morrowind and skyrim recently, and brushing up on some lore, I have a few ideas as to what I think will happen directly after the events of skyrim, and will possibly serve as the background for Elder Scrolls 6. So strap in and join me on this rollercoaster of guesstimation and imagination:
The wheel turns upon the last dragonborn and he/she achieves CHIM. Former emperor Titus Mede II is officially declared dead, after his assassination by an unknown dark brotherhood assassin. The Skyrim Civil War has reached its conclusion and either Ulfric Stormcloak or General Tullius will temporarily control Skyrim (no idea how Bethesda could make both outcomes canon without invoking the dragon-break again). The Greybeards and High King Ulfric or General Tullius declare the Dragonborn to be Tiber Septim reborn, and a new Empire rises from the ashes of the old.
Tiber Rebornius marches south towards Cyrodiil with the bulk of the Stormcloaks and Imperial soldiers, united under one banner, while Ulfric/Tullius remain behind with reserves to defend Skyrim against any potential attack. Tiber contacts the Chiefs of the Orcish strongholds and politely asks if they would join him in some Thalmor-slaying. The Chiefs send their strongest berskerkers, and send word to Orsinium to do the same. Tiber relinquishes his title of Harbinger of the Companions, and Aela the Huntress takes over and immediately begins to recruit and train more companions. The College of Winterhold send all the battlemages they have. On his way south, Tiber stops by Druadach Redoubt and speaks with King Madanach of the Forsworn. Tiber Rebornius marches south with hundreds of Forsworn barbarians, Briarharts, and hags to bolster his already impressive army.
Emperor Tiber Rebornius Septim marches into the Imperial City and claims the White Gold tower as his rightful throne. The Imperial Legions of Cyrodiil welcome him with open arms. The Emperor breathes in royalty, and changes the pleasant rolling valleys and fertile woodlands of Cyrodiil back into an unforgiving jungle, not because he loves them, but because he wants to make things as difficult as possible for any encroaching Thalmor army. Dark brotherhood assassins harass the Aldmeri Dominion (AD), unleashing a streak of assasinations on high value targets. All attempts by the Thalmor to place a contract on the Emperor are mysteriously rejected by the Night Mother. The Blades continue to grow and begin espionage missions in AD territory.
The Nerevarine returns from Akavir.
Vivec has NOT been kidnapped by Daedra, but has instead travelled to the Clockwork City in order to CHIM an old-friend back to life. Sotha Sil and Vivec emerge from the Clockwork City and travel to Mournhold to reunite with Lord Nerevar Reborn. Nerevar greets them warmly, "Vivec you s'wit! You dropped an asteroid on your own city! And why is there ash everywhere?!". Vivec gives Nerevar a cheeky smile and says: "Ever since you broke my HEART, I've been unfit to rule." In order to emphasize the punchline, Vivec quickly switches to his female form and back. Nerevar looks at Sotha Sil, "Weren't you dead the last time I saw you?!". Vivec can't contain his smile. His head is on fire.
"Yes much has changed since you were last here, Nerevar. You have a lot of news to catch up on." Nerevar shoots Sil a mischievous look. "Good. Then we have something to trade." Sotha Sil's eyes brighten inquisitively "Oh?". "Yes, my trip to Akavir was not without reward. I bring news of the Dwemer."
Back in Cyrodiil, things have heated up. A combined force of Khajiiti and Argonian warriors march through the jungles and lay siege to the Imperial City. Apparently Black Marsh have finally thrown their lot in with the Aldmeri Dominion. Emperor Tiber Septim sends pigeons to Hammerfell to let them know that they are still loved. The AD send a small force of their own to reinforce the Argonian-Khajiit army, believing that the Argonian-Khajiiti force would be enough to capture the city. But things are not as they seem...
Flashback a few months previously. A blades agent (possibly the protagonist of ESVI?) in Elsweyr has uncovered damning evidence that it was in fact the Aldmeri Dominion themselves who had orchestrated the Void Nights (4E 98 - 100) as a means of gaining Anequina and Pelletine as client states. Emperor Tiber Septim meets with the leaders of Black Marsh and Elsweyr to discuss a cunning plan...
Fastforward back to the present, the Khajiit-Argonian army ambush the much smaller Thalmor army beneath the jungled walls of the Imperial City and destroys them completely.
Much to the Emperor's surprise, Nerevar, Sotha Sil and Vivec appear inside his chambers with talks of an alliance. In exchange for the Emperor negotiating a truce between Black Marsh and Morrowind, and in exchange for the immediate withdrawal of argonian forces from Morrowind, Lord Nerevar agrees to send Morrowind's army to fight the Aldmeri Dominion. The agreement comes with one other clause however. Vivec and Sotha Sil demand to be shown the entrance to Black Reach. Nerevar guides the dunmer army into Skyrim. Thalmor spies report the army disappearing in the mountains.
Several months pass by. The Thalmor and Imperial Forces are at an impasse. The AD turn their attention towards Hammerfell. As the Thalmor Army marches into Hammerfell and begin their attack, a massive Dunmer army emerges from dwemer ruins amid the Alik'ir Desert, equiped with amor and weapons engineered by Sotha Sil himself upon the Aetherium Forge. Inside Blackreach the dunmer were also able to find an abandoned underground highway connecting to several dwemer ruins in Hammerfell. The Hammerfell forces reinforced by Nerevar's army are able to wreak havoc on the Thalmor forces.
With the bulk of AD forces tied up in Hammerfell, the Emperor and his legions, bolstered with his Orcish berskers, Forsworn barbarians, and Winterhold battlemages, march directly on Alinor. The Khajiit and Argonians join him. The Thalmor army in Hammerfell, alerted to the encroaching threat from the east, attempt to fall back on Alinor, with the Redguard/Dunmer army close on their heels. The AD army manage to make it within Alinor's walls. Practically all of Tamriel falls upon Alinor's walls. The Dovahkiin charges into battle, Aela the Huntress and the Five Hundred Companions at his back, many of whom have accepted Hircines gift. Sotha Sil calls down hell from the skies. Meteors smash into Alinors innards. Vivec flies into battle with his water spear. He is literally on fire. Nerevar, Trueflame in hand, and armored head-to-toe in his Indoril armor, fights by his side as friends of old.
Odahviing joins the party. Dovahkiin unleashes the power of the Wabbajack. Sheogorath himself emerges from the staff, with his own copy of Wabbajack. Sheogorath performs the Fish-Stick. 623 Thalmor soldiers are unable to maintain a delicate state-of-mind and are immediately driven insane. Sheogorath uses his copy of Wabbajack. The Champion of Cyrodiil is summoned. He also has a copy of Wabbajack in hand. 236 Thalmor soldiers are polymorphed into CHEESE! Sheogorath is having the time of his life.
The Thalmor realise they are fighting atleast five gods.
The Aldmeri fight desperately with everything they have. But they are hopelessly outmanned and outmatched. Dovahkiin uses the Thu'um to destroy Alinor's gates and walls. The sacking of Alinor begins. The Aldmeri Dominion surrenders unconditionally.
After the battle, Vivec, Sotha Sil, and Nerevar share drinks in a half-demolished Alinor tavern. "If only your darling wife was here to join us" Sotha Sil mumbles. Nerevar gives Sotha Sil a look of feigned confusion. "What do you mean. My darling wife has been at my side this whole time." Nerevar pulls out Azura's Star, Almalexia's purple soul swirling within. Initially dumstruck, Sotha Sil and Vivec burst out laughing as the realisation hits them. "Say hi Lexi!". The soul within turns an angry red, and Azura's Star starts to shake violently. The trio of misfits collapse to the floor with mirth.
The Dragonborn returns to the Imperial City to rule the Empire in a new golden age. Vivec, Sotha Sil, Nerevar (and Lexi) return to Morrowind to rebuild Resdayn and offer hope to their people. They establish a new stronghold at Dagoth Ur, and begin reconstruction of Akulakhan in preparation for the Landfall...