Twilight becomes alicorn of the stars. This is sort of a problem, because Luna kind of already was alicorn of the stars. Oops!

TV Tropes

Twilight Sparkle has too much magic, and it will take her decades to learn to control it. Her spectacular loss of control during her entrance exam only proves that fact. She is a danger to herself and others.

After Sunset Shimmer’s disappearance, Princess Celestia needs a new student to bear The Element of Magic to defeat Nightmare Moon. With all of Equestria at stake, Twilight Sparkle…is not the pony she needs.

Princess Celestia seals Twilight Sparkle’s magic with the distant promise of one day teaching her.

But there are two diarchs of Equestria. Amidst a lucid dream, an offer is made.

“Hello, Twilight. Would you like to be my student?”

Cover art provided by Lafiir.

Celly, Discord and Cadey have been spying observing. Luna and Twilight seem to have feelings for each other!!! The problem? They don't seem to know how each other feels!! What to do?Celestia, Discord and Cadence (Well, mostly Discord) come up with Operation TwiLuna, to get the two together in their own ways. subtletly, beautiful romance, and chaos.
I feel compelled to add "What could possibly go wrong?"

Winner of the first Twiluna group competition. (THANKS AGAIN TO EVERYPONY!!)

Celestia had a niece once, not an adopted niece like Cadance, an actual biological niece, the daughter of Princess Luna. Sadly when Luna became Nightmare Moon her daughter took her mothers side and led the their forces against Celestia.

Princess Twilight was killed during that battle, or so history tells us...

Featured on the 11/10/2015! Thanks so much!

This story is a sequel to The Code's Apprentice

Sequel To : The Code's Apprentice

The land of Equestria has been changed forever.

New monarchs sit upon the thrones of the Sun and Moon, the once almighty power of Harmony overturned to make way for a new Diarchy of Balance. Political unrest and turmoil have engulfed the land, and at the very center of it all, Princess Celestia - renamed Tia Sunbeam - seeks atonement for the crimes she committed in the name of Eternal Harmony.

With much of her power sealed away, Tia must live amidst the citizens of Ponyville as the world changes all around them. As Ponyville works to rebuild following the struggle between student and teacher, they must cope with the edicts of a new government determined to bring equality to all creatures, beliefs, and ways of life - even those that were long since banished to the dark places of the world.

Tia Sunbeam will struggle with her own deeply held convictions on the way the world should be, versus the way it will now become. She will face a truth she has long since denied, and realize the true meaning of Harmony. And perhaps, most importantly of all... She must make some Friends.

Cover art by SinSays at http://sinsays.tumblr.com/ - Go spend some money with her! :D
Oh, and go check out my new side story - Rise - It's going to become relevant here!
(character tags are not comprehensive! Some are being kept secret! ;3)

For a thousand years, the Code of Harmony has ruled Equestria, and the Code of Dissonance has sabotaged its works at every turn. Now the war for control over the land will come to a head, and one unicorn's decision may decide the fate of all.

For though she does not know it, Twilight Sparkle is the shining example of the Code of Harmony. Her teacher, Princess Celestia is Harmony's most powerful master and she knows her enemies are hatching a plan that threatens to cast down the Harmony of her throne and all she has sacrificed to create.

What she does not know is her foe seeks a new Apprentice to turn to his dark ways. A unicorn with seemingly limitless potential for magic, whose thirst for knowledge may yet lead her down a dark path, who is known to be such a faithful student of magic, eagerly seeking the truth no matter how forbidden it might be...

Sequel: Code of Harmony

This story is a sequel to The Luna Cypher

Twilight sets out to learn more of the ways of the world, and soon decides that the whole planet is disorganized, insanely dangerous, and desperately needs to be fixed. And she's just the mare to do it... with a little help from her friends.

Also, there are sky-pirates!

Book 3 of the Alicorn Adventure series.

A few extra, slice-o-life chapters are here: On the Rocks

This story is a sequel to The Celestia Code

When the monsters created by the dark crystal engine escape their prison, it's up to Princess Twilight Sparkle to deal with them. She gets help and advice from Princess Luna, but things don't go as planned. In fact, when a pony who relies on logic and rationality tries to navigate the stormy seas of emotional entanglement while simultaneously saving Equestria from a horde of evil creatures intent on destroying everything they encounter, nothing goes as planned.
AUDIOBOOK by VisualPony
WARNING: Major spoilers in the comments.

Book 2 of the Alicorn Adventure series.

Goodreads Page
100% Approved by Twilight's Library!

The Badass Twilight tag applies more than ever, but please note the new tags.

This story is a fairly direct continuation of The Celestia Code. You might be able to follow it without having read The Celestia Code first, but it's better if you do.

The sequel is here: The Twilight Enigma

Most excellent pre-reading/editing done by the erudite AcademicPony and the dapper statoose!

Princess Twilight Sparkle discovers a centuries-old mystery hidden in the Royal Archives. Her investigation leads through layer after layer of deception and misdirection, setting her hooves on a path that seems to be leading to a mysterious secret. Along the way, she learns that some friendships can be very, very strange.

Book 1 of the Alicorn Adventure series.

AUDIOBOOK by VisualPony
Featured at the Royal Canterlot Library
TV Tropes Page (Courtesy of Sandtalon)
Goodreads Page (Courtesy of Facedeer)
Equestria Daily Page

WARNING: Major spoilers in the comments. Takes place several months after the end of season four. (Not a Da Vinci Code crossover, BTW!)

Do you see a dark tag? No, you don't. Not dark. Well, okay, there are a couple of dark-ish bits, but that's all. Not enough to warrant a tag. Speaking of tags, there really ought to be "Badass Twilight" and "Friendshipping" ones.

Sequels and side-stories (and more) can be found here Offprint and AO3, and a hardbound two-volume set of all that can be ordered from The Ministry of Image.

My editor for the first edition of this story was Gogito, and I can't thank him enough for all the hard work he did. My editor for the revised version was PresentPerfect, and he did a terrific job of getting the story into excellent shape.

Celestia's morning has been difficult enough, between hearing Twilight's confession of love, and then breaking her heart as gently as possible. Mere moments later, her private chambers are rocked by the shockwaves of a time-traveling spell. This marks the arrival of Celestia's alleged lover from five thousand years in the future, Princess Twilight Sparkle—or as she insists on Celestia referring to her, Glitter-Flanks.

Armed with thousands of years of knowledge of which buttons to push and how to make her squirm, Glitter-Flanks has traveled back to this date for one singular purpose: to establish the closed time loop that sets the two of them up as a couple.