Okay, so! My vision for a demon race is for the model to not only gain muscle mass, but also to gain features(horns, etc.) and to enhance upon existing features, too. For example, the character starts with a short set of horns, and as she grows, so too does her horns.
Here's a list of features that I can think of:
*A scaled prehensile tail.
*A elongated tongue.
*Demon horns. At later stages of growth, perhaps they, too, could emanate fire.
*Glowing eyes, perhaps with a serpentine slit pupil.Perhaps the glow emissions would intensify as growth progresses? Maybe with black sclera, too.
*Glowing hair. I personally think that all the glowing bits ought to be the same color, but you do you.
*Demon teeth. You ought to distinguish this race's teeth from a an animal's if you decide to implement a beast race, and certainly from the orc race.
*Sharp nails or sharp fingers. Perhaps they could retract or reshape themselves during civil discourse?
*Toe talons or toe claws.
*Black nails and toenails.
*Wings, but I'm not sure how you'd make that work. If you do, I suggest scale wings or black feathered wings.
*Assertive posture. She should look like she owns the place, confident or overconfident, with perhaps some vanity added too.
*Extra arms, if you want to get nuts.
*Red, purple, or black skin pigmentation. Maybe her skin changes color as she grows?
Something of note is that red fire is not the hottest there is: that belongs to white fire. If you include light into the demon race design, perhaps you could have the light change color to signify a growth of power.
I once watched this video about how red, blue, and purple have come to be associated this different concepts. It's called "The color of corruption - how purple is used in video games" by Razbuten. My focus is fading, I'll return to this later.