Image imported from derpibooru 
Original Description:
 
Celestia and Luna are very interesting and important characters to make up headcannons for. I personally like trying to interpret and piece together canon content (or even bits posted by Hasbro or FIM crew), and boy that can sure be tricky for such big characters such as the princesses. I think the biggest clues are from the Legends of Magic comics that show events further back than the show does, and The Journal of the Two Sisters. There doesn’t seem to be many clues as to where they came from and their origins, as well as the origins of alicorns in general. From the journal they both recount their adventures in Canterlot as fillies, and from the wording used in the book it alludes to there being other alicorns. Celestia and Luna know that they age slower as alicorns, as if taught by other alicorns or as if this is some already known knowledge as learned from previous alicorns. What’s frustrating is that it’s brought up several times that Starswirl approached the two sisters to rule Equestria, but approached them from where? Where were they when Starswirl approached them? Had he seen them before? Were they living with other alicorns? Celestia writes how Starswirl says that the two are perfect rulers since alicorns stood for everything… yadda yadda, so Starswirl knows of alicorns, but then why Celestia and Luna? Luna writes how her and Celestia were taught the royal canterlot voice, but from who? And the fact that it is royal means that where they came from has some powerful connections. Ugh, just so many questions and not a ton of answers. From all this though, I think I can build something pretty plausible, maybe I’ll expand on it and make some drawings in the future, I’m thinking and researching all this as I type this up right now, lol.
 
Celestia and Luna are natural born alicorns, and from that comment Celestia makes about Flurry Heart, it’s just because there have been no alicorns born in Equestria, since the sisters were born before Equestria was created. Celestia and Luna were born and raised with alicorn knowledge, such as the royal voice and the knowledge that they age slower than the regular pony. They also had an abundance of other knowledge, learning about the tree of harmony before they saw it for themselves. Somehow, other alicorns had disappeared, though it doesn’t seem to be anything traumatic as their writings don’t indicate any sort of thing. Celestia and Luna spent their younger years in Canterlot amongst themselves, playing pretend as royals and making it by themselves, perhaps stumbling across other ponies and creatures. Word makes it to the newly forming Equestrian nation, especially to the ears or Starswirl the bearded. Only those well versed had heard of alicorns before, and hearing that there were two young alicorns among Equestria, Starswirl finds them to be perfect candidates for the new rulers of Equestria. Along with being able to study and research true alicorns up close. Our story aligns with cannon, growing up, earning their cutie marks and taking the reigns of the celestial bodies, defeating
 
Now I like to follow even the smallest scraps of cannon… until there’s some idea I like following better. Luna and Celestia are in all ways, the dark and light. Luna’s inconsistences with her cutie mark can be chocked up to her being being fully intertwined with darkness. Celestia and Luna’s unique abilities come from this, and effect their appearance. But I also just reeeally like Luna being part bat-pony, or at least sharing some characteristics. This comes from her transformation into Nightmare, fully becoming darkness. Also makes sense with the fact she can communicate with animals, or at least feels fitting.
 
In terms of what I changed about their designs, I added juuuuust a bit. I reordered Celestia’s mane colors to look a little more pretty, along with adding a yellow gradient cause it looks pretty, but also represents her age. I gave Celestia markings to mirror Luna’s too. And speaking of markings, something you can see here is them without their embellishments. I wanted to draw them here since I changed them up a bit, especially on Luna. They have what are called The Element’s Scars. Celestia’s is more severe than Luna’s due to her taking control of all the elements to banish Luna. Their necklaces are tailored to cover these specific markings, especially Celestia’s.
 
 
And there’s my ramblings! Sorry for the absence, I’ve been dealing with some shit and school ^^