Not really an AU anymore considering the latest tweet from Lauren Faust which all but implied that the only reason Nightmare Moon escaped was simply put because Celestia let her out.
 
This can be taken to the extremely negative when you consider that the only reason Luna turned into Nghtmare Moon was out of loneliness and emotional pain caused by her own sisters’ ego. And that Celestia didn’t try bothering to meet her sister halfway when it comes to ruling night and day that caused her to turn into Nightmare Moon and force, no, give Celestia a reason to banish Luna altogether and rule Equestria by herself. And not once in 1000 years did Celestia ever bother trying to go up to the Moon to apologize for what she did, or to let her out even though she had the power to do so… nor did she even apologize when Luna did return, only demand for Luna to accept her friendship without question.
 
This makes the whole outburst between the two of them in From Dusk to Dawn and the “Daybreaker” sequence in A Royal Problem even more painful to watch: especially when the Daybreaker is actually more in-line with Celestias actual emotions…