Wow, did this ever get away from me. And my poor discount computer, trying to handle six or seven layers at that size. It was not happy with me.
I kinda designed this like it was a storybook illustration, perhaps an illumination painted on leather or parchment. Save for the final pair of Luna and Celestia in the foreground, who are demonstrating their vast, cosmic powers by casually stepping
into the painting.
And there's some headcanon here too, or is it wishful thinking? As with many of us, "Magical Mystery Cure" burst all my alicorn theories like big, pastel-colored, pony-shaped balloons. So I latched onto our new worldbuilding tidbit, that alicorns were once ordinary ponies. It occurred to me that there was a G1 pony who was white with pink hair and had a solar name: Sundance. I
love the idea that Celestia used to be an earth pony (it seems wrong to restrict ascension to unicorns), though this does hinge on ponies being able to change cutie marks.

Anyway, it seems I still have a long way to go with using color and arranging elements in a picture, but hey, it was fun.
ETA: Featured
here on a new site,
Equestria Confidential, devoted to fanwork. Thanks, guys!
2nd Edit: Featured on EQD's 799th Drawfriend! Wow. Just... wow.
Done in ArtRage, using chalk.