I had this idea about how alicorns “cycle” through rule. The eldest alicorn rules, while the younger ones defer to her. The very first reigning alicorn was Faust herself, who was then succeeded by Celestia. After Celestia would be Luna, then Cadence, then Twilight, Flurry Heart, and future alicorns.

Alicorns don’t actually live forever, they “die of old age” when their magical power gets so immense that they become the essence of the things they represent. Faust literally became the universe, and then several thousand years after Luna’s return, Princess Celestia “ascended” to become the sun itself. By then Equestria had hit an age of technology, and under Princess Luna’s leadership, the ponies begin to expand into space. Not a few centuries afterwards, Luna also ascended and became the moon. This left Cadence as the eldest alicorn with Twilight, and Flurry Heart to help her rule Equestria and its new star systems.

Twilight, always fascinated with knowledge and discovery, left Equestria and became Princess of the Stars. Cadence stayed behind with her daughter to watch over the homeworld of the ponies, reigning as Princess of the Earth (“earth” as in earth pony, not the planet).

Mi amore Cadenza, Princess of the Earth:

• While Twilight rules over the Equestrians in space, Cadence watches over the billions of creatures still on the homeworld of Equestria. After Princess Luna’s ascension, Cadence moves to Canterlot, which was by now the heart of the massive Equestrian ecumenopolis (think planet-wide city like Coruscant from Star Wars).
• Cadence pretty much looks the same as show Cadence since she was already pretty mature when she appeared onscreen. She still wears the same regalia, except several sizes larger (she’s about Celestia’s size now)
  ** Over the years, Cadence’s hair has the ethereal quality that all alicorns achieve when they reach a certain level of power. Her wings have also become huge as a result of her increased magical power.
  ** After spending thousands of years in the Crystal Empire, Cadence has picked up crystal growths all over her body like the crystal ponies. She still lacks other earth pony traits.
  ** Flurry Heart took over her rule in the Crystal Empire and became the new Princess of Love.
• Cadence dies a few centuries after the creation of the Equestrian galactic empire. She sacrifices herself to save the planet from an alien threat as one final act of love for her ponies. Flurry Heart takes over for her mother as Princess of the Earth.

Twilight Sparkle, Princess of the Stars:

• Twilight rules the Equestrian galactic empire, which consists of star systems, trade routes, and satellite colonies. As also the Ambassador of Equestria, she meets new races and civilizations to spread friendship and harmony across the stars. (once the princess of friendship, always the princess of friendship)
  ** Her castle and capital is one giant space ship/port/city that travels from place to place as necessary. Twilight designed it herself to as a place for friendship and union between diverse civilizations. It contains the most diverse population of ponies and other creatures in the entire galaxy.
• Her regalia holds the entire set of the Elements of Harmony, with the Element of Magic set in her crown and the five others in her necklace.
  ** Unfortunately, most of Equestria has forgotten that she was once the princess of friendship, and few know about the existence of the Elements of Harmony. Most just think they’re for decoration.
  ** Twilight hasn’t used them in a long time, but she still wears them in memory of her friends. They are still with her, even thousands of years in the future.
• As she’s gotten older, more and more features from the 3 tribes has mixed themselves in. Her hair is now ethereal and it has the yellow streaks from her rainbow power form. There’s also stars in them, but I was too lazy to draw them in.
  ** Twilight’s horn has grown to insane lengths due to her continued pursuit in magic and science. Her unicorn tail has shortened a bit as other features got mixed in
  ** Compared to Cadence, Twilight’s wings still remain small and are primarily used more for gliding. You’d think that after thousands of years, she would have learned to use them properly, but with the invention of the space ship, she has even less need to fly.
  ** Her fetlocks have become ethereal like her hair as well. She has developed a bit of earth pony strength and endurance, which she usually uses to hoof-build her complex inventions and go for days without sleeping.
• Twilight has taken many apprentices over the years. A few have also ascended to become alicorns, who were then given their own systems to govern.

Even after all that time and space, Cadence still remained the princess of love till the end, and Twilight the princess of friendship. Some things really do last forever.