In response to Grifcannon:
The reason Twilight did not have her cutie mark during the Cadence flashbacks is hopefully an animation mistake.
But even if that is not the case, it can be easily resolved by Celestia knowing of Twilights destiny before she got her cutiemark.
How could she know? Maybe she sensed it in Twilight in meeting when twilight was to young to remember. Or maybe she knew what year Twilight would be born and had Cadance sit for all the fillies born that year. Or Cadance was to sit for all unicorns with a strong bloodline? Maybe Celestia in all her subtlety suggested to Cadance the idea of being a foalsitter, knowing that she might encounter the heir. Or maybe there is simply a prophecy. The point is, we can get around those problems by wriggling a bit.
In response to Codex:
Oh how I would love to cut your belly open, so that I may learn the secrets of you gut.
What makes you think in terms of decades or years? I would rather have said weeks or even days...
You know it's during shifts of power that an empire is the most unstable.
Hmm maybe your reason are all the things Luna and Celestia managed to do together before the falling out?
Defeating Sombra and Discord and the like in just a couple of months is perhaps unrealistic?
Hmm interesting, but what if the "evils" if Equestria are especially active during the time of a powershift?
Then that could mean Luna and Celestia put under a lot of pressure during a short time. Making perhaps, for the ideal environment for a falling out between the sisters.
I believe a thousand years is a short time for a nation to survive if it is ruled by an immortal god in a world were wars are fought with apple pie.
Also ponies seems to be better people in general than people.
In the story of Hearth's Warming Eve we are told that the unicorns raised the sun and moon. Would not the simple explanation be that two of the unicorns held the cutimarks of godhood, without necessary being the rulers of their group?. (It was a different time after all)
I like to believe that the Hearth's Warming Eve story took place many powershifts ago. And that maybe at some point the cutimarks came to a alicorn who resolved, that if possible the powers should go on to alicorns.
When Celestia could not find a second alicorn, Twilight the unicorn became the second choice.(however she would need guidance)
Like you Codex, I am having a hard time to let go of this theory of mine. Maybe cause its promising something grand for season 4.
I really wish to see Twilights true succession of Celestia. And in that succession, the death of Celestia and Luna.
An ending worthy of any Disney movie.