A good story doesn't have to dissect the motivations of every character for the reader... but the main characters, especially the Point of View ones, should have reasons for what they do. No one springs from a vacuum; everyone comes from somewhere.

In creating this group, I hope to collect and highlight stories that go that extra mile in how they portray characters and/or how they explain their behaviors.

Flim & Flam tried to take ownership of Sweet Apple Acres. Why? In Eros Dreaming, Flim & Flam aren’t just con-men - they’re unicorns with apple-based cutie marks. Not science, not magic, not stars… apples. Like one of the largest and most famous earth pony clans. The pressures they faced from their own family and in trying to pursue their dreams & talents… that’s what drove them to try and take over the farm.

Blueblood’s supposed to be Canterlot’s most eligible bachelor… so why did he act the way he did at the Gala? In Blueblood’s Just Zis Guy, it’s not because he’s a jerk - it’s because he’s cripplingly shy and socially inexperienced; like a combination of Flutteryshy and Twilight before she came to Ponyville. The events of the Gala happened just as they were shown, but not because Blueblood meant to be rude; he honestly didn’t know any better.

In The Corner of (Our) Eyes, Ditzy doesn’t have wonky eyes “just because” - there’s a reason for it.

The stories collected here should all, in one way or another, show a character in a way that gives them greater depth than a running gag or fan-created meme. What made them the pony they are today? Or what else do they do besides the single thing everyone else has them doing?

Why shouldn’t every pony be a fully rounded character, with virtues, vices and reasons for how they act?

So just to clarify, you want stories that expand on the quarks (not ignoring them completely)?

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Expand on them, ignore them... really, I'm looking for stories that treat characters as more than just cardboard cutouts that only say the same 3 things over and over again.

I suppose another way to think of it is "... and here's why" or "... but there's more!"

Why does Bulk Biceps only say "Yeah!"?
What does Lyra like besides humans?
If Blueblood is a jerk, why do the magazines call him "most eligible unicorn bachelor"? Are all the other unicorns WORSE?

Because fan-quirks aside, the only things that are canon are the stuff that happened on-screen.