I was giving this idea some though, and I think I might have come up with something that can tie the anthology under one banner, yet give the writers plenty of freedom:What if?
This might just be me, since I've always been interested in alternate universes, but what if the theme is showing how events could have played out had varying variables played out different in the show? Say, for example, what if Gilda had been more humble and apologized for her attitude? What if Discord pulled a switcheroo at the last second, using his powers to turn one of the mane six into a mirage of him, while he switches places? what if Trixie saw through Twilight's ruse and opted to keep the Alicorn necklace?
All stories would be unique in form, but follow the theme of alternates. The would could even be shown to end in some of these stories, and it would not affect the others, as they are all "what ifs" by nature. Naturally, there could be a stronger narrative that ties some together, like perhaps a pony that can hope between realities to see events first hoof, or how the fate of one universe suddenly affects another in subtle ways.
I think this would be an interesting idea to act on, but that's just me. What do you guys think?
>pic related. An alternate reality story done right (for the most part).
As for where this could be done? Fimfic sounds like a good place to look for candidates, but you could reach out elsewhere as well: the other chan, the IRC, and a few emails.