It took years, and Twilight hadn't even expected it to happen anymore at this point.

Out of all her friends, Rainbow Dash was the first to ascend.

Relevant blog posts:

• "Dragon Science", Immortality, and how to save the ones you love in a world of magic.
• Sleepovers at Canterlot Castle: Seven ways Twilight could see her friends MORE than just "once a month", while staying canon-compatible
• What Celestia and Luna are up to next...
• A thought: Cozy's/Chrysalis'/Tirek's reformation does not need to wait until the distant future
• Example of how a "6 months in stone" reformation-fic could go.
• The best way to save Twilight's friends and create a happy future, without playing the immortality blues
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• Influencing the canon status quo through the IDW season 10 comics

A villain reformation story doesn't need to take place decades in the future.

A few months in stone can be enough. This would still be plausible:

In the future, we never saw their statue - they could've already been reformed ages ago.

Twilight's big on reformation - don't you think she'd have tried sooner rather than later?

Here's an example of how a "6 months in stone" reformation-fic could go.

I'd hate to lose the opportunity to see positive stories written about these characters interacting with the ponies we know in present-day Equestria.

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There are so many ways Twilight and her friends could see each other more often than just "once a month".

E.g., if you can build a permanent two-way gateway between two universes (Equestria Girls), then building one between Ponyville and Canterlot Castle should be trivial. It'd almost be, as if Twilight had never left town.

And let's not even get started on Luna's ability to allow ponies to share dreams...

Their moonly meeting is simply when they all meet up for the whole day, without interruptions, is all.
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What are Celestia and Luna up to these days?

Vector sources: Celestia's cutie mark,Luna's cutie mark, Cutie Map from a screencap

The Call to Adventure

Free from their duties, Celestia and Luna are summoned on a friendship mission.

Vectors used: Luna, podium, Background

Vote Luna for Mayor!

Bored, Luna decides to run for mayor of Silver Shoals as a hobby.

How hard can running a town be after running a country?

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How to save the ones you love in a world of magic:

[image source]

Dragon Science

Dragons live for hundreds, if not thousands of years.

Other creatures usually don't.

One dragon is planning to change this.

"...What is it about dragons that makes us live so much longer? And how do we give it to other creatures?"

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[vectors used: Shining and Cadence by 90sigma, background by Mandydax, image compiled by me]

I will wait for you

Shining Armor, unlike his sister, wife, and daughter, is not an alicorn.

He knows he won't live forever.

But if he lives long enough, he just might.

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Find these and other story scenarios on life extension, and how to plausibly save everyone, here.
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What's the "best way" to tell a happy future story about Twilight's friends becoming immortal, without playing the immortality blues?
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TL;DR: Basically, just give them alicornhood at the start of the story, then see how that affects their lives. Set the "outliving friends and family" issue aside for the B-plot, mention Twilight already has several life extension projects being worked on.

More alicorns = shared responsibilities = more spare time for Twilight, more time together.

Overall: a happy slice of life piece, rather than just "an end". Would have to be a multi-chapter story though to really explore it.

DO NOT ABANDON YOUR PRE-SEASON 9 STORY IDEAS!

WRITE THEM ANYWAY. Treat season 9 as the "alternate continuity".

Your readers will understand if you put a "diverges from canon after season 8" sticker on new stories published even years from now, because it's all about reader immersion.

To this very day, you can still read a fic with black changelings in it and instantly understand the inherent dilemmas a disguised changeling faces, instantly accept a continuity where the "metamorphosis" does not occur, and a different solution must be found. You can do this, because we've had a lot of pre-s6 changeling fics, and many that still continued afterwards to this day.

And if they can keep going for years after, then so can you!

And I do mean that your readers will understand a "ignores season 9", not just a "takes place before season 9". We've had 8+ years of lore and fanon before season 9. If you read an older story, your brain just flips a switch.

Good examples for deliberate canon breaks include a certain quote from an s9 fic by FanOfMostEverything (though I won't give away which fic):
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"On the subject of lessons and alternatives, are you truly sure you wish to pursue this 'School of Friendship' idea, Twilight? It seems a waste of your considerable talents."

"Well, we are getting quite a few new students [...]" Twilight laughed. "Besides, what else would I do? Try to fill your hipposandals?"

Both sisters smiled at that, or at least drew back their lips. Celestia cleared her throat. "Heh. Yes. How ridiculous."

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And Kris Overstreet, who wrote an entire chapter in "Changeling Space Program" that's all about distancing himself from the s6 finale by spinning it as a ludicrous nightmare Chrysalis had.

I hope this was helpful and inspiring to some.

We can make this a better future, both with, and without outright contradicting canon.