Well, I was watching Lullaby for a Princess and did have a tought... Not sure if exists already...

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In the normal story, Celestia use the elements to banish Nightmare Moon to the moon for a thousand years... What if she decided instead to banish herself?

Is the final of the battle, Nightmare Moon is in front of Celestia and she can see how Celestia begin to charge the Elements, but instead of shooting them to Nightmare Moon, she shoot them to herself saying a simple.

"Goodbay Luna, I'm sorry for everything"

After that, she is sended to the sun a thousand years. Full of grief after watching that, she can't do anything except cry. 3 days and 3 nights crying until in the end, she return to normal and decides that the adoration of the pony is not as valuable as her sister, so she decides to hide herself, at first thinking of not even raising the sun never more but then she decides to continue with the sun/night for her sister even when she hide herself.

And so, she and her sister become legends, forgotten in time. Until a thousand years happen. Celestia wake up in the ruins of the old castle but there is no trace of Luna in a world that don't know about her or her sister, she begin to search for her. At the same time that Luna discover that Celestia is back but she feel to bad that don't want her sister to discover her but at the same time she want to meet again with her after a thousand years.

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That is the main idea

5734132 Interesting idea, but it needs a bit more polish. Three days is pretty short of a time for her to grieve, considering that she had planned to kill Celestia in the first place. Three hundred years would be a much more likely number that would still be possible, though that runs into the issue of ponies freezing via eternal night. In addition to that, it kinda requires Celestia to hold a combination of idiocy and optimism that don't really fit her show canon very well and REALLY don't fit her fandom canon at all.

This one has some similar ideas. It veers off from yours as Celestia banishes herself. In this one she manages to pull the Nightmare from Luna into herself before leaving.

http://www.fimfiction.net/story/351321/the-solar-steward