This idea in short Celestia vs Daybreaker;

Premise;
Whilst in the midst of calming the Nightmare that plagues Starlight, for a fraction of a second, Celestia accepts that maybe this is what she maybe one day could become. Daybreaker notices this and dives into Celestia amidst the chaos. Starlight awakes in shock to find Celestia unconscious but soon finds he reeling in pain as Daybreaker separates herself from Celestia. Daybreaker has become reality, but at the cost of both Celestia and herself only having half the power each. It will soon be dawn and Daybreaker tells Celestia if she wishes to have what she wants she will meet her at the place where she was forced to fight 1000 years ago, alone.

(For this next part of the idea, I urge you to listen to this

and let your imagination run with the scenario;)

Celestia meets with Daybreaker in the throne room of the Castle ruins. Daybreaker boasts about how things could have been if Celestia didn't choose the elements but her own power. Taunts her of how she and the world was better off without her sister, to which Celestia threatens her to not speak ill of Luna, again. To it, Daybreaker replies; "If you don't want me to say the truth about your sister! Then, well," Daybreaker paused as a sadistic grin formed, her horn charging with power, "You're going to have to silence me!"

Daybreaker throws the first blow as Celestia starts the battle remaining on the defensive but forces herself to stand up against her darker side fiery maned maniacally laughing Princess began to have an echo to her cackling. The two clash in a battle of wills, ideals, beliefs and what they truly feel is right and what should have been done. Columns of fire blaze through ruined exterior as the two sides of the sun clash hoof to hoof, beam to beam, head to head. The fires of conflict, setting the ruins of the old Castle alight (if that's possible) and even spreads to the edge of the Everfree Forest (If you want to go that far). Who will win this clash of the eternal flames? Will Celestia triumph over her darker self? Or will Daybreaker swallow the full brink of the power of the Sun and dominate the world with her unrestrained power? You decide.

So, thoughts?

P.S. If anybody wants to go through with this, all I ask if for credit in their story description.