The question is simple, what do you suppose became of her after her mother became ruler of Equestria?

she will become Equestria's dark sword

Equestria s hidden assassin
Equestrias 1st ruler to be wed.
Or... she just becomes more awesome in some way.

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For me, Nyx would wander the world protecting and helping any creature in need. She'd make it a point to come home three times a year to visit Twilight, Spike and the CMC before going out again, and she would continue like that until her final day.

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In my story Goodbye my Child she became co ruler with her mother, they all outlived their friends being alicorns but I only had them have a longer lifespan rather than immortality. Nyx took over the dream realm for Luna.

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She looks pretty, I'd say she'd be the one in charge of the guard or something.

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Either straight-up captain of the guard, or some sort of independent Lady Protector of the Realm.

In my point of view, she will wander all over Equestria protecting everypony from harm, or lingering evil. Her only times of returning to either Canterlot or Ponyville (to her now restored former Castle), is either to report new threats that are lingering, or visiting family and loved ones. In other words, a lone-mare wanderer following her creed.

Nyx will grow up and then use a spell to regress and turn herself into a filly again, maybe staying near Twilight so that she don't have any responsibility like stupid paperwork and when she grow up again then she will regress again with her marefriend Flurry as both of them live as fillies using cuteness as a weapon

I actually think that she may stay with Twilight in the castle. Being the reincarnation of the incarnation of the night princess, Twilight and Nyx will probably replace Celestia and Luna respectively. The reason I see it this way is because of the fact that in The Road Home, Nyx starts showing signs of having inherited Luna's dream walking ability. So when Rainbow Dash called having Luna's dream walking ability, she didn't take into account that the role was already taken.

I suppose the dream walking ability doesn't necessarily have to be a royal duty or have to be at the castle to do it. But that's my reasoning. Perhaps she could've stayed in Ponyville to be with her fell CMCs. Or perhaps in the Nyxverse, the CMCs started traveling Equestria, perhaps sent by the map, to deal with cutie mark problems.