I brainstormed my own interpretation of a Nightmare
Cadance.

Fimblveter

Fimblveter is one of the disasters before Ragnarok, for 3 seasons long Fimblveter, a long horrid winter, ravages the land. not only bringing a cold blanket of death, but also the death of all relationships. lover turned to hate, familial relationships falling apart into cold feuds.
The crystal empire is all about love, and its various forms, so if the empress fell to darkness it would reverse to a cold hatred. The Crystal Heart would be worthless, and Fimblveter, the mare who used to be Cadance, would only coldly march in silence across Equestria freezing all in her wake. no hatred, no laughing, no plan for world domination, just winter
If the state of the empire is supposed to reflect the state of equestrian then it would be disastrous if it were to fall, Fimblveter would only leave frozen ponies in her wake.

As far as her visual design, as often as i see icicles, and crystal like hard jagged edges for ice characters in the mlp fandom, most people forget that blizzards, and other frost things actually have very organic swirls, and patterns that are pleasing to look at. So I made Fimblveter's mane a blizzard in constant motion that would likely bleed into any blizzards she would make from herself.

The wings are basically Nightmare Moon but more feathery.

The colors are also basically just snow, and the colors of untouched snow for camouflage purposes, or to signify her being the storm.