I hope Sunset Shimmer replaces Twilight as the new pony in Ponyville. Sunset Shimer recently moved from the [insert city name here] to Ponyville on an exploration, and discovered a love for friendship that she had neglected because she had been to intreseted in planetary movement. She's not a Crystal Pony because crystal ponies are earth ponies and SS is a unicorn.
I think it's very likely that this pony is her sister: [link]
No two other crystal ponies we have seen have a day/night motif, so they could possibly be like Celestia and Luna, except that the night pony is the older sister. And because of the planet in the other mysterious mare's cutie mark, I don't think she's a crystal version of Luna.