I think I’ve finally settled on a design I’m happy for with this character, so here, have a first look at them; Pyre, adoptive child of Celestia, and biological child of Ember.
 
She is very talkative -a bit too much for her mom, but she just nods along with whatever Pyre’s chattering on about. She’s very outgoing, and stupidly brave. And she likes to think she is very clever and witty, but most of her plans and logic is… you know… child logic. She might grow up into a tactical mastermind or something, but right now the guards just pretend to not see her covered in a sheet as she’s trying to sneak into the pantry with the gems.
 

 
Her story starts with Ember. Before the Gauntlet, she wasn’t exactly sought after during breeding seasons; even being the daughter of the Dragon Lord, she was very small and physically weak for her age. Not something dragons find very attractive. After she became Dragon Lord, practically every dragon threw themselves at her, hoping she’d pick one of them. So after the next breeding season she laid three eggs -A very small clutch for a dragon, but that’s probably for the best considering her small stature. Two hatched without problem and their magic made them imprint on her, (A hatchling imprints on whichever parent’s magic most resembles their own) much to the dissapointment of all the potential fathers, but there’s not much to do about it. Better luck next season.
 
The third egg didn’t hatch, and Ember wrote it off as a dud. It sucks, especially with such a small clutch, but it’s life, it happens. She still kept it, deciding to have it as a sentimental keepsake of her first clutch. That is, until she happend to mention it to Spike. He gave her new hope when he told her about how his egg was hatched with magic; he didn’t have enough magic himself to hatch, so Twilight essentially flooded him with hers until he was able to break out. The catch was that by doing this she altered his magic signature to be the same as hers, thus making him imprint on her instead of whoever his parents were.
 
Ember figured that she’d rather have her hatchling be imprinted on someone else rather than it not hatching at all, and immedietly asked Twilight if she would take it. Twilight and Rarity reluctantly agreed; normally she wouldn’t hesitate, but she and Rarity was currently dealing with toddler Moonstone’s uncontrollable magic and Spike’s teenage rebellion, so they were worried if they’d have enough time to devote to a hatchling. (Which, if Twilight remembers correctly, takes a lot of attention and time to care for) Still, it made sense; she was one of the few ponies who had experience raising a dragon, it would be good for the hatchling to be around Spike, and Ember already knew and trusted her. Plus, she was the only pony who knew the spell.
 
That’s when Celestia interjected; she was certain she could learn it. She also had experience helping raise a dragon; it wasn’t like Twilight did much of raising hatchling Spike when she was a foal. That was either Twilight Velvet and Night Light, or Celestia when they needed a break. The hatchling would still have the advantage of being close to Spike, and she would have more time to devote to it than Twilight and Rarity. (Plus, Celestia just really likes children and being a mother.)
 
Ember agreed, and thus, Pyre was hatched. As expected, she imprinted on Celestia, which made her instictively try to get away from and attack any dragon whose magic wasn’t similar to hers. It was… disheartening for Ember, but she stood resolute that this outcome was better than an unhatched egg. Celestia let her choose the name before she left to return to her other babies in the Dragonlands. Once Pyre got older and lost more and more of her hatchling instincts, Celestia made sure to take her to visit Ember and her siblings when she could, and to invite the dragons to the castle whenever Ember could get away from her duties.
 
Like a lot of dragons, Pyre is intersex. (Dragons are essentially a conglomerate of mutations piled on top of a lizard, so intersex individuals are extremely common in them compared to other species; along with things such as them suddenly switching sex upon molting, or just randomly during their lifetime. Dragons don’t really bat an eye at this, aside from maybe rethinking their mating strategy if they want hatchlings.) She currently identifies as genderfluid; mostly female, sometimes agender, and rarely as male. This might change as she grows older though, as she’s still trying to figure out what feels right. She goes by she/her and they/them.
 
She’s currenly in her teens, and just recently got her wings. For a dragon, this means that she’s a small child, even if no longer a baby. A lot of dragons force their kids to leave when they start molting, but ponies aren’t exactly keen on kicking out children, so Celestia is her legal guardian until she reaches adulthood… at about 50-60 years old. Another very good reason for non-ageing Celestia to be her mother.
 
(A lot of my reasoning here is based on my dragon headcanons, but that’s even longer, so feel free to skip it if you don’t feel like reading even more )
 
Tldr; Ember had dud-egg, Celstia hatched it using same spell which hatched Spike. Baby imprinted on her, lives with her now.
 

 
Oh, and a first look at Celestia in my nextgen verse. I didn’t alter her design much, just gave her some more yellow in her color scheme (as any self-respecting sun-themed character should have). And very faint freckles, to tie her design just a bit more to my version of Luna ^^