Interestingly, the destiny thing has been something I'd thought about in the past. I thought about an idea for a short fanfiction designed to teach some of the basics of rationality, wherein Twilight was totally clueless about how to fix Starswirl's spell in the Season 3 finale. Twilight would be forced to learn the basics of rationality in some fashion, specifically the portion about mysterious answers, noticing that "destiny" didn't actually ANSWER anything, forcing her to clarify her true answers. By working on that, she discovers the true nature of Starswirl's spell, and is thus able to counter it and restore her friends back to normal. (Not sure what it would have been, but I was thinking it would be based on mind magic. Cutie marks would be based on belief, and the spell tricked them into believing that their destinies were something else, which caused their cutie marks to change: Cutie marks aren't caused by destiny, belief in destiny causes cutie marks.) In this case, Celestia and Luna actually know the score, but unlike mortal ponies, they WERE created for a purpose (By a being that remains unknown, being outside the scale of the fanfiction.) and they DO have raising the sun and moon as their destinies. So their beliefs that make their cutie marks appear as what they are are actually correct.

Then I came up with this transhumanist idea, and decided it was better than my other idea for a rationality-based fanfic.