The Call to Adventure

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

Vector sources: Celestia's cutie mark,Luna's cutie mark, Cutie Map from a screencap
[art by shamanguli]

The idea of the Immortal Goddesses of the Sun and Moon going into quiet retirement never sat right with me.

Sure, nopony is more deserving of getting time off for a job well done - but the idea of their retirement felt like Celestia and Luna were becoming less "relevant", less "important"; as if Equestria's lore itself, the legend of the "Eternal Goddesses of Sun and Moon" we were told in the very first episode of MLP, was suddenly shattered.

And that ... that just felt wrong.

...So it'd be nice to see fanon of them taking on a new role in Equestrian society further down the line.

Protecting Equestria, leading expeditions into the unknown larger world, going on world-saving adventures themselves for once. Going on friendship missions. Give their lives a new meaning.

Who knows... "peaceful retirement" might get boring after a while.

Here's a few story ideas I came up with.
(posted in separate comments since it'll be easier to distinguish replies)

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?

[art by 90sigma]

“By the Powers of Sun and Moon!”

Celestia and Luna, free from their duties, and somewhat bored with retirement, decide to use their alicorn powers to become “super-heroes”.

Their identity isn't exactly a secret, but it's fun to play pretend, and traveling Equestria and helping / saving ponies is its own reward.* It also gets them more in touch with the everymare.

But darker forces are stirring in Equestria's underbelly – and they've set their sights on the two sisters... surely, now they are retired, that must mean they are weakened!

Will Solaris and Moon-Mare be able to stop them before it is too late?

^{*Maybe don't focus on just Ponyville this time. ^__^;;}

Vectors used: Celestia sitting on her throne, Twilight's cutie mark

Returning to the throne

Princess Twilight, ruling monarch of Equestria, is summoned on an urgent matter out of the country, and it's clear this will take a while.

"Celestia, Luna, I hate to ask this of you – but can you fill in for me?"

It's been 500 years. It's only for a month, right?

Everything's gonna be fine.

[artwork by foxy-5]

Space-Princess Luna

Luna is most intrigued by Princess Twilight Sparkle's endeavors into space travel and plans for colonizing other worlds.

And come to think of it, she's had an awful lot of free time lately...

Here's something for if you want to go meta:

Two parallel Equestrias make contact

The triarchy of Equestria makes diplomatic contact with a parallel Equestria. To Celestia's, Luna's, and Twilight's surprise, the other world's Celestia and Luna are retired, leaving Twilight the sole ruler of the nation.

Celestia, Luna, Twilight, and Celestia, Luna, and Twilight have a chat.

How things are, and how they could've been solved differently.

Lastly... here's something I had cobbled together (from existing vectors) before the s9 premiere had even aired:

Vectors used: Twilight , Celestia , Luna, Background

True Equals

“You needed room to grow. Yet had we raised the possibility of someday returning to the throne, ponies would not have taken you seriously as a ruler... Will you accept our aid?”

Also be sure to check out FanOfMostEverything's ideas on the subject here.