I based this on an old unused version of Rarity I wrote a bio for. The story sort of fizzled out in the early stages.
 
Here I will go on a rambling tangent and tell you the tale of how Rarity became Princess Rarity of the Crystal Empire:
 
Basically, one of the timeline’s divergences is during the events of Boast Busters. The alt-Twilight had to face the Ursa Minor, but also its far larger and more powerful mother. The sheer strain of sending both back to their cave proved too much, and she died from magical burnout and exhaustion.
 
To those that hate Snips and Snails, yes they were severely punished for their foolish actions.
 
The Element of Magic, in the vein of the Green Lantern ring, chose a good-aligned Sunset Shimmer as its new bearer. Storywise this was necessary so that Discord could be defeated.
 
A number of years down the line, the Crystal Empire returns just as in the show. However, in this universe, Cadence is not a princess or an Alicorn. After the defeat of Sombra, the empire was without a ruler, and power struggles and borderline civil war began to occur as some individuals sought the throne for themselves.
 
A chance discovery in some geneology records, proved that Rarity was descended from Princess Platinum, the last ruler of the Crystal Empire. While was she was executed by Sombra, her heir was secreted out of the kingdom, surviving to continue the royal bloodline.
 
Rarity reluctantly chose to leave Ponyville behind to help save the Crystal Empire. Carousel Boutique was passed on to her good friends Suri and Coco (the former being good-aligned instead of evil).
 
However, she was met with some opposition. The Crystal Ponies hailed Spike as their hero and savior, and thus wanted him to become prince. Only then would they accept Rarity as their princess.
 
Spike had a somewhat easier time leaving Ponyville, as the town brought about too many painful memories of Twilight. Thus he packed up and accepted his new role as the Crystal Prince. However, he and Rarity do not have a romantic relationship due to their age difference, and are simply very good friends and equal co-rulers.
 
Both of course remembered their friends, and always welcomed visits, and an invite was sent each year to the Crystal Fair.
 
As for a future heir, they eventually adopt an infant filly, whom they named Ruby. She is the daughter of Blueblood, who rejected her after her mother, one of his servants, passed on due to labor complications.
 
For his cruel cowardice, Blueblood was exiled to the Badlands by Celestia, until such as time as he could become a better pony.