I was reading a comic about what another commentator called, 'a sleep paralysis angel', which got me wondering:
What if Luna's and Nightmare Moon's roles, if not personalities, were reversed?
Edit: Maybe not completely, but significantly enough for them to be clearly heroic/villainous. Depends, obviously.

7904418
Then Celestia and Day Breaker's roles would also be reversed.

Well, you'll probably start an imposter syndrome story.

Luna plots to make sure the night is loved, then the eventual Nightmare Moon takes over. But with a different moniker like "Luna Moon", for example. This magical take over is same as canon.

The difference now is that this "Nightmare" uses common sense and is not as eager to take over the world. So when the ponies soon love her night, Luna deems it is time to remove the "Nightmare" magic.

Except, she can't. Her "Nightmare" is an entirely living entity, just like Tantabus. "Nightmare" does one thing, but Luna tries her best to stop her, unsuccessfully. She would try to communicate to her sister that she is trapped, but nothing comes from it. Eventually, the ponies would praise "Nightmare" more than Luna.

A thousand years later, "Nightmare" and Celestia still rules the nation without much issue. They are still the Element bearers but they don't spread friendship as well as what Twilight would have. Speaking of Twilight and Sunset, Celestia doesn't really try to groom them into alicornhood, but at least she has Cadence.

By this time, "Nightmare" is basically the default Luna, and Luna herself might have forgotten that she existed. Not sure if there would be a catalyst for this, but maybe encountering an infiltrating changeling would wake her up.

Tl;dr
Luna inadvertantly made Nightmare an internal nightmare rather than siccing her on the world. This is a psychological horror story; Monkey's Paw situation.

7904431
Ooo... The awakening happening during the, 'Canterlot Wedding' would lead to some interesting complications and issues afterwards...