This is a story inspired by Hopeful Ink Hoof's "multiple alternate Sunsets" story:

Specifically, it's inspired by the Sunset Siren character from that story.

In this case, it has the Mane Six and the Pillars switching roles (as well as Spike and Stygian and Sunset and the Dazzlings)

The basic story is this: Twilight (in this case, Twilight the Great) was Celestia and Luna's mentor more than 1000 years ago - and, more than 1000 years later, Starswirl (way too young to have the beard yet [in fact, at the present time in the story, he's barely in young adulthood] , though he DOES have the blue cape and hat) ended up becoming Celestia AND Luna's shared personal student.

As with canon, Twilight was pretty asocial at first until she met five not-so-ordinary mares and a young dragon (the latter of whom would end up becoming like a little brother to her) who would change her life for the better - it just happened more than 1000 years earlier. And Celestia and Luna met Twilight's group of friends multiple times over the course of the time she mentored the two of them.

Of course, Twilight and her friends also saved the world literally a dozen times over - the first time was from a siren going by the name of Sunset Shimmer.

And, fortunately, when Spike was feeling overshadowed by Twilight and her friends, they DID talk things over enough to prevent him from going over to the dark side. Something Celestia and Luna took to heart as they talked out their own issues without either of them going bad.

Unfortunately, they weren't as lucky with a certain chaos god going by the name Discord. All attempts to reason with him had failed and Twilight and her friends were forced to use their most powerful creation - the Elements of Harmony - for the last time to turn him to stone. They succeeded, but NOT before Discord managed to conjure up a vortex that trapped the Mane Six in dimensional limbo, meaning it became a case of Mutually Assured Imprisonment.

Almost a thousand years later, at a fateful entrance exam for the Princesses' School For The Gifted, a young colt named Starswirl, despite his youth, showed a level of magic power equal to about fifty fully trained adult mages. Power he had an extremely hard time controlling and ended up conjuring a shadow demon that possessed Stygian (another colt in a nearby testing room). This forced Celestia and Luna to get directly involved in order to get the situation under control.

Realizing the potential Starswirl had if he learned to control his power, Celestia offered to make him her and Luna's personal student (silently hoping she wasn't making a mistake like she did with her LAST personal student - Adagio Dazzle - and HER two friends Aria Blaze and Sonata Dusk).

By the time the two alicorns and Starswirl figured out how to successfully separate the shadow demon from Stygian without hurting Stygian, Starswirl and Stygian had legitimately bonded - to the point where Stygian could honestly be considered Starswirl's first (and, for a little over a decade, only) friend.

Though Stygian was nowhere near as powerful as Starswirl, he WAS an even better researcher (which is saying something considering Starswirl was far from a slouch in the research department). Unfortunately, Stygian was ALSO pretty badly bullied by everybody except Starswirl, the Princesses and the guards. Starswirl was often quick to leap to his friend's defense (to the point where he sometimes got in trouble for magically invoking karma on Stygian's bullies).

Ultimately, Stygian and Starswirl found out that Discord was soon going to escape from his stone prison and tried to tell the Princesses. Celestia and Luna reassured the duo that they would do what they could, but, at the same time, they should get additional help and make back-up plans.

To that end, Starswirl and Stygian headed to the nearest town to Canterlot - namely Ponyville - to see if they could find anybody who might stand any chance at all of helping them stop Discord should Celestia and Luna fail.

While there, they meet Rockhoof and Flash Magnus (who were co-chiefs of security for Ponyville possessing incredible strength [even by Earth pony standards] and an indestructible shield respectively) as well as Mistmane (who had a great talent with plants) and Mage Meadowbrook (who was a great medic and excellent with potions) and Somnambula (who had a knack for finding ways to overcome great obstacles and keeping others positive).

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And, a couple of additional notes:

The reason Twilight didn't finish her masterpiece spell WASN'T because she didn't understand friendship, but rather because she understood it TOO well (i.e. she realized the spell had the potential to be entirely too dangerous and didn't want to needlessly endanger her friends)

In addition, Granny Smith, Big Mac and Apple Bloom are actually Applejack's DESCENDANTS rather than her grandmother and siblings respectively.

Similarly all of the Mane Six's canon families are their descendants rather than their parents, siblings, etc.

Plus, Spike (who WASN'T imprisoned in dimensional limbo like the Mane Six were) went on to become the Dragon Lord (and, in fact, HAS been the Dragon Lord for just a few years shy of a millennium at the start of the story) and lead the dragons into a firm alliance and friendship with the ponies (a few particularly jerky teenagers aside).

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Ultimately, though, I can't think of enough details to make the idea work. Anybody who CAN think of enough details, though, is more than welcome to it.

Just remember to credit Hopeful Ink Hoof for inspiring the idea.

7011612
Damn, I really like this, hope someone takes it up. AU fics these past few years have gotten pretty stale, with the vast majority just being, [Insert Character] taking the place of Twilight, or vice versa. We need to Make AU's Great Again, people gotta get more creative in how they use characters, and change the roles in their worlds.

7011915
Thanks. I'm glad you think it's an interesting idea. Hopefully, it'll get adopted by somebody with the skill to do it justice.