Been rereading RH Junior's Parting Words when I had a sudden epiphany: All that stress of being Celestia's student, all that studying, all those tests from both the school and her mentor, Twilight must have had quite a stressful foalhood. All that stress at such a young age, it's no wonder Twilight's so neurotic and and anal-retentive, and a gods-damned miracle that she's not suffering from a more serious medical condition.

But what if she did?

What if, before the start of the series proper, while Twilight's still a filly, all that stress actually causes the likes of a psychotic break, or even a full-blown stroke or heart-attack (just a few examples, feel free to use what you want.)?

The idea wouldn't be about whether or not she recovers (she does, at least to a point), but how much things would change if such a thing were to happen. How would Celestia react? How would Twilight's family react? How would Canterlot, and possibly all of Equestria react? How much would Twilight change? How much would Twilight's relationship with Celestia change? How much would the future change?

Just had a thought: You know how Pinkie Pie threw a surprise party for Twilight on her first day in Ponyville? Imagine that if Twilight had a heart condition due to a heart-attack during foal-hood. I can see Twilight trying to keep herself calm, less she has another heart attack, and Spike coming to Twilight's defense and scolding Pinkie.