Come to think of it, I've got a better concept for Sombra and The Empire. (We're going back in time to when nothing beyond Season 2 has been written)
* The Crystal Empire was built over this fount of crystal energy.
* This fount was used to create powerful relics, and it played a role in the creation of The Elements of Harmony.
* King Sombra took control. The Crystal Ponies are the results of him experimenting with crystal magic; trying to re-create The Crystal Heart. This lets Sombra be a foil to Twilight and Cadence: he represents a corrupted version of Twilight's pursuit of knowledge, and he sacrifices others, which contrasts the "self-sacrifice" idea.
* Princess Cadence is a Chosen One, destined to rule The Empire. Notice how her Cutie Mark depicts The Crystal Heart. She got her CM after a vivid dream about Sombra and The Empire.
* This idea of "destined rulers" would have set up Alicorn Twilight a lot better. The whole "pressure of destiny" idea becomes something Cadence and Twilight bond over.