Nu-Fall Plot Sketch

20XX: Moore's Law has broken down as transistors reach the atomic level, and quantum computing's qubits have added their usefulness. Russian computer scientist Ivan Mikhail publishes a landmark study confirming that efficiency of both human-written and algorithm-written code decreased as processing power increased in the last century, resulting in only a linear increase in effective processing power. With examples of clean, efficient legacy code harder and harder to come by, engineers everywhere curse Mikhail's Law as a confirmation of evolutionary laziness.
21XX: Small groups make extraordinary profits in mining, fuel sales, and tax/data havens across Sol. Some have seen the coming storm and fled Sol entirely on sub-C ships. Most have not. A group on Earth develops a strong AI, which in a few hours consumes Sol and repurposes it for computation. That AI later becomes the Terminus. Those who have fled in time have established hardscrabble colonies and know little of the fate of Sol, calling themselves the Diapsora. The population of the human race is down to the millions.
23XX: An entity calling itself the Patron contacts the Diapsora one by one, offering stability and growth in exchange for political unity. Most accept, and the lives of those who do not are short. The Edictus is born, and with little contact to the outside world the Patron-Emperor of the Edictus establishes Service Arms and issues a steady stream of tame decrees. These come to be renown for their foresight and positive effect decades or even centuries later.
25XX: Science cracks the secret of negative mass matter creation, enabling FTL of multiple types to be built. Negative mass axions are used to fuel FTL drives, of which Alcubierre-type bubbles and Einstein-Rosen-type wormholes are the most prevalent. The Edict of Free Travel enables any Edictan citizen to colonize an exoplanet, gas giant moon, or build an O'Neill cylinder station in barren star systems. The explosion of colonization is unprecedented, with every human with even the slightest whim to become god of their own star system builds or rents a drive and heads off with nanoconstructors.
26XX: Mankind has spread all across the Orion Arm of the Milky Way, with hundreds of worlds. Technology level and population vary wildly.

EMPEROR IS A MORTAL AND DIED/DISAPPEARED DURING FALL, BUT PLAY IT UP LIKE HE'S A GOD OR AI FOR LULZ.
The Edictus is infamous in human history as the only major empire to allow itself to fall apart. The Romans, Britons, Americans, and Jovians all fought tooth and nail to preserve their empires until the bitter, dying end, but the Edictans simply put it to a vote after their Patron disappeared. Anyone who wanted out got out, and far from being a test of loyalty vast swathes of the Edictus reformed themselves. There was the Kantus, populist rebels long a thorn in the side of the Service Arms and led by someone calling itself the First Citizen. They were a wildly diverse group, filled with genos (Genome-Altered Citizens) and their thralls. The Edictus were filled with a vast number of virtual reality addicts who stayed on to keep collecting their dole of data and processing power. Precious few patriots joined them in realspace. The Arthran were the final of the Big Three, ex-military and machine-minded technocrats who earnestly believed that the Emperor was just out for tea and holding fast to a united, technologically advanced government would give Him a nice base to work with when He returned. All of them were wrong in their own unique ways.