[Plank]

I would say it depends on the theme and how demons work in your particular continuity. Succubus are all about transformation, because they are constantly taking the form of a persons desire to seduce them easily, even if it is just in dream form- attacking while they are asleep. Does she siphon off enough energy through sex to just outright kill the victim, or does she just leave them weak as a kitten?

Staying true to classic mythos, succubus and incubus are children of Lilith who is a greater demon, though originally a human woman, so technically they could look perfectly human. Spade-tipped tail, cloven hooves(thanks to Baphomet's classic appearance), and horns are the default demon traits.

If you are looking to give a more frightening appearance, try twisting things around a bit, making her true form seem deliberately 'wrong'. Like she has wings but they are upside down so they shouldn't work- but do work anyway. Or maybe as a result of their innate power to take on the appearance of your specific fetish, perhaps she has no true form- her true form can just be an ever shifting, distorting set of traits that never quite settle on any one face.

In one of my own very old stories, the main character unintentionally summoned a succubus who ended up being his companion through the rest of the story- she was easy to walk around with in broad daylight because whenever anyone looked at her they seen their own ideal vision of a person, not her true self. Because they are ultra-sexual and female, if you add traits I would make them small, dainty hooves, very long but narrow horns, and thin cord-like tail. Also, if a succubus uses her spade-tipped tail to direct you to her holes from behind, the reverse-spade looks like a heart-shaped pointer. :P