@Kyler: My thoughts exactly.
Okay, headcanon time. This will probably all be disproved in S4, but whatever. Here we go:
1. Each race/country has its own magical artifact/set of artifacts linked to it. Equestria has the Elements of Harmony; the Crystal Empire has the Crystal Heart.
2. Said artifact is linked to the line of succession in each kingdom. Cadance was identified as rule of the Crystal Empire because the crystal heart is in her cutie mark, and Luna and Celestia's claim to power was validated through their use of the Elements of Harmony. When the elements first switched hoofs in the pilot episodes was when Celestia confirmed that Twilight (And possibly the other ponies; it remains to be seen) would eventually become a princess.
3. This means that the rulers of Equestria are chosen according to their ability to manifest the elements; a leader who isn't honest, kind, loyal, generous, and having a sense of humor won't ever be able to sync with the element of magic and trigger the ascension. The reason Twilight ascended is because she manifests all the elements rather than being tied to a specific one.
4. Any member of any pony race can sync with any element, but each at least one member of each race has to bear an element. This means that it's possible for a unicorn or EP to be the element of magic. However, as alicorns represent all three races, at least one of each must bear an element.
5. Celestia and Luna are special cases in they were somehow already alicorns before syncing with the elements (Celestia says Twilight did what nopony else did). That's why she and Luna were apparently able to bear the six elements between them; they manifest the traits of all races. Either they were born alicorns because of their inherent ability to manifest the elements (perhaps the Elements ascended them without them knowing exactly how it happened) or there's some other method. I haven't quite hammered out those details yet.
edited 16th Feb '13 4:30:49 PM by JapaneseTeeth