I've been consider how I can expand on my idea from "The Day Before", which considers that while it never happened, the other Mane Five lived out their lives in an alternate timeline. (If you've seen the TV show Fringe, think it's fourth season premise.)
I've got the idea of a blocking out a reason to explore these past "false" memories from the standpoint of Twilight's research as part of studying the Elements. From there, I've some interest, if not complex, ideas of trying to revisit certain episodes considering the characters in their swapped roles (e.g. "Too Many Fluttershys").
With that, I've already envisioned the backstories (even through the pre-cutie mark - and an aspect I'm also considering). Some examples that I'm clear on:
- Pinkie follows from a few fan comics, marrying into the Apple family via Big Mac, gaining the apple cutie mark from her baking skill; subsequently she's forced to take care of Sweet Apple Acres after Big Mac suffers an injury.
- Rarity I've envisioned as being born to two pegasi and showing aptitude in weather control (just --- not great) and hence she's sent off to Cloudsdale University - along with a cloud-walking spell - to get better at it, though, as shown by Magical Mystery Cure, still has problems with the thundery ones.
The other three (AJ, Fluttershy, and Rainbow) I've got ideas for but need to flesh out better.
A key factor here, at least in writing "The Day Before" is that I want to keep a hint of what the real talents are in writing these - an element that something is amiss but they can't place it. For example, Rarity would care about how she looks, color choices, and hinting towards her fascination with literal weather patterns. But still would never second guess that weather control is her cutie mark.
As I want to envision it - everypony else in the universe remains the same and fulfills the same role unless it affects one of the Mane 5's backstory. (such as Apple Bloom being Pinkie's step-sister and having no relationship with Applejack).