Elder Scrolls VI: The Return of the Dwarves
Background: The Aldmeri Dominion truly dominates much of Tamriel. Men and "lesser mer" are increasingly subjugated, and the world looks bleak for those not born of high elven blood. Through not only politics and economics, but also magical mind control of councillors and yarls, the Altmer control seats of power throughout Tamriel.
As the story begins, it appears that the Altmer are the least of Tamriel's worries. In an instant, an ancient power appears from out of nowhere and completely decimates Altmer control. Massive, towering colossi appear near the major cities of the world, quickly occupying each provincial capital. The key cities in the Summerset Isles, Cyrodil and Hammerfell are each controlled by a unique mechanical colossus -- each apparently designed to entrench the landscape and architecture of each city.
Our hero awakes in a jail cell beneath a major city in Summerset, startled by the shaking earth around him/her. A voice appears in the jail cell, guiding our hero out from the prison on a journey to discover who is behind this new terror -- and where they came from.
The Antagonists:
The Dwemer have returned from their disappearance in terrifying form. Led by the great architect Kagrenac, a series of massive mechanical colossi overtake each great city in Tamriel. There appear to be no living dwarves aboard each colossus, only centurions and other dwemer automatons. They quickly take each city, as if they were designed for centuries for this purpose alone -- tailor made to each city's architecture and economy. Given their technological mastery, they even see the Altmer as a lesser race to be controlled.
They appear to be impervious to attack and have no clear entrance. They are completely formidable, and Tamriel feels hopeless against them. Those that serve them survive, those that do not are consumed into soul harvests to power the machines.
The player ultimately discovers that each colossus is controlled by an ancient dwemer lord, a boss who is hard-wired into the machine but will disconnect to fight our hero aboard each colossus. Each lord's defeat brings down the Colossus, freeing the city beneath it.
The Resistance:
Our hero is guided by the Psijic Order and The Blades to fight the new Dwemer threat. The blades organize a resistance in each city. The Psijic Order guide our hero to unlock and attack the Dwemer force in each city.
How does the Psijic Order help us against each colossus? We're guided to an old, magnificent Dwemer ruin in Hammerfell that is virtually untouched by time. Only the Psijic Order believed it actually existed. At its base is one of the finest works of Dwemer technology, a time travel device powered by an Elder Scroll. With Psijic tinkering, it allows its user to virtually travel through time into the conscious of an ancient Psijic wizard: Sotha Sil, god of the tribunal and combatant in the War of the First Council.
The game is then played in two times. The ancient era, where the player as Sotha Sil works to uncover the Dwemer plot and create a safeguard for the future Tamriel. And in modern Tamriel, where the player uses that safeguard to fight against the colossi.
The game takes place in modern Summerset and Hammerfell -- and Morrowind during the War of the First Council.
(cool DLC idea: a remnant of those bloody Thalmor figure out how to reactivate a colossus after its defeat, and they take ahold of the Imperial City using its remains. Think Death Star II: Thalmor Edition)
TLDR: Ancient Dwemer lords reappear as massive colossi, take key cities around Tamriel. Hero must travel through time to before the Dwemer departed, through a spiritual atunement with Sotha Sil, god of the Tribunal and a Psijic wizard. Player in modern Tamriel uses an ancient secret to unlock and destroy each colossus, and then defeat their great architect Kagrenac once-and-for-all.