Character designs I think are neat
A fire-themed set of siblings. Both have messy, almost wavy red hair. The elder sibling primarily wears brown clothing (representing a torch), and the younger primarily wears gray and has a hoodie with a particularly boxy hood that they like to completely cover their head with (representing a lighter, specifically Zippo style lighters).
A mirror-themed character whose appearance reflects that of whoever is looking at them. The exception to this is darkness-, ghostly-, or otherwise invisible-themed characters (infrared light, UV light, etcetera), who see a translucent, glassy figure.
What if dryads/ents, except burnt and ashy and generally themed around plant-life/flora after a forest fire? Like seriously, this sort of design is my jam. Give me burnt-undead plant-entities that crackle and moan and have a faint, flickering fiery glow that can be seen seeping out of their joints and mouth and eye-holes!
Any sort of "duality" or "opposites" theming. Like two ice/snow-themed characters, one who represents fun stuff like snowmen and ice-skating and snowball fights and the other just flat out looks like they fell through an iced-over lake and/or froze to death.
Characters themed after stuff like dice, cards, coins, slots, spinners, roulettes, and other things of that nature. Bonus points if a villainous character with this sort of design has elements that suggest "tampering" or "being loaded."
Non-horrific horror-themed characters. Like. Cutesy Vampire teens in their victorian nobleperson garments desperately trying to catch up on popular culture ("Vat is an 'updog?'"), or young werewolves that just look like slightly-feral kids with a lot of hair and sharper teeth. You go on down to the local doctor's office and you're greeted by a doctor and a nurse who are coated in blood, have an uncomfortable habit of staring unblinkingly, and really didn't think through the decor all that well, but they're both genuinely there to provide medical help. That's the sort of vibe I'm going for here.
admittedly I'm a sucker for gooey or rubberhose-themed characters. Like. Stretchy! Bendy! They can puddle! My monkey brain is awash in the dopamine chemicals!