Friendship Is Magic: The Next Generation begins thirty years after the birth of Princess Flurry Heart, and the story is… a bit darker and more complicated than My Little Pony: Friendship Is Magic before it. It delves into the backstory around Alicorns, where they came from, why Discord is the only Draconequus, and who is the prisoner in the deepest darkest cell of the cosmic oubliette that is Tartarus…
(And admittedly, FIM:TNG does take some liberties with the events of MLP:FIM, mostly in regards to Equestria Girls and all episodes after and including The Cutie Re-Mark. That will become more apparent when it comes to the character bios for Sunset Shimmer, Flash Sentry, Starlight Glimmer, and others.)

This is Gold Dust, age 22, known as Goldie to some and Dusty to others, daughter and only child to Sunburst & Captain Flash Sentry. She lives with her dads in Shining Oak Library, a tree born of a sapling found near the remains of Golden Oak Library. Sunburst being such a bibliophile, it was only natural since Twilight Sparkle declined to leave her now more homey castle that he would choose Shining Oak Library as his home. And curiously, though Shining Oak Library was an oak tree, it grew with a glistening semi-crystalline bark and it rose in the span of only about five years, as if to make up for the loss of its progenitor.

Gold Dust's special talent is in light- and lightning-based magic, particularly in being able to move fast, although not so fast as Dad Flash's sister Lightning Dust for whom she is partially named. One spell she has crafted allows her to create a long-distance light-clone of herself from a shaft of sun- or moonlight. She is working on a spell to allow herself to travel in a shaft of light, but Aunt Lightning doubts that Goldie will be able to ever outpace her.
Dusty's cutie mark on its own

Dusty thinks that books are fine, but (heresy incoming) feels that there is only so much you can learn from them. Action in the field and testing your hypotheses by getting your hooves dirty is much more her speed. Her friend Shamrock Apple much approves of this approach, as does Sunset Shimmer, one of the mothers of another friend Daybreak Dream.

Gold Dust is… a bit of a proactive mare when it comes to competition in magic. She definitely inherited that streak from Aunt Lightning, and not often in a good way. She will often butt heads with ponies who she perceives as either competitors or inhibitors of her ambitions. Her chief goal is to become Twilight Sparkle's next apprentice in magical teachings and learn the highest studies and deepest secrets of magic. This is not to say that she is a cold or malicious mare. She only wants to help other ponies, although she can sometimes lose her way if "ends justify the means" takes too much prevalence.

When they chose to have a foal, Flash & Sunburst naturally needed to find a surrogate mare to carry the child for them. Sunburst's childhood friend Starlight Glimmer was a possible candidate, but in the end they chose Flash's older twin Lightning Dust. She had reentered her brother's life a moderately more humble mare, having apparently been demoted in Wonderbolts Academy for endangering the lives of other ponies. As an act of good faith to her little brother for their separation, she volunteered to carry their child. As such, she was a big part of her niece's life, stoking the fire lighting her goals. Although, she may be setting her niece up for a hard fall if she proves to be too ambitious…
Gold Dust refers to Flash Sentry as Dad or Daddy and Sunburst as Papa.

Gold Dust's eye color comes from the gem on Sunburst's robe. Her coat color comes from Lightning Dust's mane color. Her mane color comes from Lightning Dust's cutie mark, and from Sunburst's painted-horse parts. Her cutie mark's colors come from Flash Sentry's mane color, Sunburst's gem, and Lightning Dust's cutie mark. The shape of her muzzle is derived from Saffron Masala's, because in my book it reflected on her being the progeny of two dads.

Sunburst first met Flash Sentry when Princess Celestia set up the Dusk Guard around Princess Twilight Sparkle's castle following the events of The Cutie Re-Mark. The bookish Unicorn got a huge crush on the Royal Guard as soon as seeing him, and Flash himself found the resultant stumbling quite charming. Finally, somepony who was bumping into him, not vice versa! Princess Cadance could immediately see the love connection that they shared and coerced them into sharing time together. Asking Sunburst to deliver certain books to Captain Flash now and then, setting Flash on guard duty for Sunburst as he read books… though Flash often raised his eyebrows at this, it became clear what the Alicorn of Love was doing. In due time, the two stallions became an item and moved in together at Shining Oak Library.

…oh yeah, Flash Sentry and Lightning Dust as siblings. See, when their dads Ice Crystal & Thundercloud were expecting a child together from their surrogate, they had chosen the unisex name Lightning Flash for the foal. But to their surprise, twins were born, a filly first and then a colt about two minutes later. They named the filly Lightning and the colt Flash. Lightning was always very speedy and controlled in her direction compared to her more klutzy and bumping-into-everypony-he-meets twin. She was the lead flyer, and he was her wingpony. They pushed their separate beds together. They were kind of like the Pines twins from Gravity Falls, always together and planning to keep things that way.

One day in their youth, she won a major race among the other young Pegasi, with admittedly quite strong and determined competition. In a blast of lightning magic, she bolted to the finish line and earned her cutie mark. Flash cheered his sister on, declaring that the second-place racer had "eaten Lightning's dust!". She liked the sound of that, and appended Dust to the end of her name from then on.

Their dads were very proud, but the competing fillies and colts were incensed at the loss. The next day they cornered Lightning Dust on the playground, and they had every intent of plucking every feather from her wings. Flash hesitated on the sidelines for several moments, scared that his clumsiness would only make things worse. But hearing his sister's cry of pain from the first primary being pulled drove him into action. The bullies were blown back from Lightning by a gust called forth by Flash, who stood steady between them and his sister. A shield of swirling air surrounded her to protect her from all sides. He dared them to approach her again. He let her down already, and he would NEVER leave her side. The cruel foals withdrew, and Flash let the magic cyclone pass in a sigh of relief.

Lightning Dust darted to her dads in a panic. The wing needed medical attention with the loss of a feather that was forcefully preened far too prematurely. At the hospital, Flash stood like a sentinel at her door. Then, there was a blooming light from his flanks. He too had earned his cutie mark that day, a shield with a lightning-bolt. Crystal & Thunder were immensely proud of both of their children, saying that "Flash is a sentry for his sister". And like Lightning Dust less than an hour before, Flash redubbed himself Flash Sentry. And though both were showered with love and affection for their cutie marks for weeks after, Lightning was unable to fly until that particular feather grew back. She grew to resent Flash for not having saved her sooner, and a gulf began to grow between them. It started when she pushed her bed off to the other side of the room. They began to argue more, and eventually she took a separate path home from school than Flash. She would not depend on anypony anymore, and anypony that couldn't keep up just needed to eat Lightning's dust. Without his sister, Flash Sentry needed somepony to protect to fulfill that calling in his cutie mark. His aspirations led him to the Royal Guard, and eventually he (literally) bumped into a particular Princess of Friendship…

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Since my Duchess Skystar was uploaded here, I decided to get on with uploading the rest of the Friendship Is Magic: The Next Generation cast.