Twilight Sparkle was Celestia's chosen heir, and under her rule Equestria was destined to prosper. But then her friends passed, as mortal ponies always do, and she was left to rule alone. The years were not kind to her after that.
Humanity's space exploration ultimately took the form of billions of identical probes, capable of building anything (including astronauts themselves) upon arrival at their destinations. One lands in Equestria. Things go downhill from there.
Thanks to the love bomb at the Royal Wedding, Princess Celestia is revealed to be a Changeling Queen. How will Equestria, deal with this? And why is Celestia a Changeling Queen in the first place?
As a joke during his original rule, Discord sent a group of ponies to a distant, inhospitable planet and left them alone. Thousands of years later, their descendants notice a star in their sky orbiting inexplicably around its planet.