In the shards of NightmareMoons armor the essence the very spirit of nightmare moon is still embedded. Then hoping for reformation of the dark alicorn, Luna and Celestia employ discords power to restore Nightmare to a limited physical form. She is then treated by a phyicologicst, maybe falls in love, and is tried to be befriended by Luna and Celestia, almost like a protical sister

2502062
Definitely would be an interesting redemption story (interesting enough that I would read it). Though, as to whether it would be good or not depends on the author's skill. I could see it going either way.

What's this psychologist nonsense? Everyone knows that the only possible solution is for Luna to dump the unreformed Nightmare on Twilight in the middle of the night without providing any guidance or information. And also without telling her sister.

Something about the situation presented here is too....organized. It's almost like something a sane person would do.

There needs to be conflict. If you're going to go the route of Celestia agreeing to bring back Nightmare, there first has to be the story of Celestia becoming okay with this. I mean, what exactly is bringing back Nightmare Moon intended to accomplish? Who's advocating this and why? I'm all for bringing back and reforming Nightmare, but not if the sole reason is that the other Princesses happened to have joint custody of the idiot ball that day.

2502062 This seems like they are purposely trying to create Nyx. I would very much like to see this done by a good author as well. Would she be brought back as a filly or full grown mare? Or perhaps a teenager? Romance would be a real possibility but that's fraught with more problems than the implimanation of a Mid East peace treaty.
Yes, I would very much like to see this happen.

Or maybe Discord decides to bring back Nightmare Moon of his own free will without either of the Princess's knowing.

2502062 It could work, but they would need a good reason to regenerate Nightmare Moon. Some greater threat the existing alicorns cannot face perhaps. Depending on how you look at it, an independent Nightmare Moon would be closer to an elemental than to a living alicorn, and thus would have other vulnerabilities and invulnerabilites than them.

That does sound like fun.