HELLO I USED TO WRITE SOULMATE AUS HERE IS MY LIST. you may get varying mileage out of it. also, none of these are my original ideas but i compiled this list in like 2018 or something (D:) and apparently didn't bother crediting most of them so unfortunately their creators are lost to time, but i've included citations where i have them. anyway. hopefully this can be of some use to you :3 happy writing
Color Marks:
You see everything in shades of your soulmate's eye color until you meet them.
You see everything BUT your soulmate's eye color until you meet them (and look into their eyes)
You see color when you first touch your soulmate.
Colors start appearing the closer you get to your soulmate.
Your eyes change color with your soulmate's hair (for example, if they dye it).
One eye is your eye color, one eye is your soulmate's eye color.
You stop seeing colors once you meet your soulmate.
The colors you can see correspond to your soulmate's current emotions.
Anti-Aging Mark (you stop aging at X years old until you meet your soulmate)
Countdown Mark (You have a timer on you that counts down until you meet your soulmate)
Date Mark (You have dates on your body of your soulmate's birth date, the date you meet them, and optionally the date they die)
Word Tattoo Marks
Your tattoo is your soulmate's name
Your tattoo is the first words your soulmate says to you.
Your tattoo is the last words your soulmate says to you.
Everything your soulmate has been called is on your body somewhere. The more often they've been called that, the bigger it is. (ex. names, nicknames, adjectives, insults)
Your tattoo is something your soulmate will say to you at some point (not necessarily first or last)
Non-Word Tattoo Marks:
You have a tattoo somewhere on your body that tells you what your soulmate is most passionate about.
Your tattoo tells you how old your soulmate will be when you meet.
You have a tattoo that is the same as your soulmate's (a) in the same place as your soulmate's, (b) in a random place, (c) that runs across your body to meet your soulmate's when you first meet / touch
You have matching tattoos that grow and expand the closer you get to your soulmate (physical or emotional distance)
You have half a tattoo and your soulmate's eye color has the matching half
Song Mark (When you get a song stuck in your head, your soulmate is singing it / your soulmate is listening to it)
Injury Sharing Marks
You share your soulmate's injuries (and do / don't feel the pain) (as in, if they skin their knee you also skin your knee)
You get your soulmate's injuries (as in, if they skin their knee it's actually your knee that gets skinned, and NOT theirs)
Doodle Mark
Whenever you draw something on your body, it appears on your soulmate's body
Whenever you draw on someone else's body, it appears on your soulmate's body
First Touch Mark (There's a black stain where you and your soulmate make first contact and it turns colorful once you meet)
Red String of Fate (A red string on your pinky finger connects you and your soulmate)
Body Swap Marks
You and your soulmate swap bodies on your birthday / new years for a period of time
You and your soulmate body swap randomly for random amounts of time.
You and your soulmate swap bodies and can't swap back until you touch
Hot & Cold Mark (You feel warmer or colder depending on how physically close you are to your soulmate)
Thought Marks
Your headvoice (the 'voice' that narrates your thoughts) sounds like your soulmate
You can communicate with your soulmate telepathically
Dream Marks
You have dreams about your soulmate's daily life (either regularly or randomly)
Whenever you dream, you look through your soulmate's eyes
You dream about the most memorable part of your soulmate's day
You and your soulmate can interact in dreams
Emotion Marks (Your emotions either get more or less intense the closer you get physically to your soulmate)
Movement Mark (You and your soulmate randomly start copying each other's movements; think Ruggie's "Laugh With Me" except involuntary on everyone's part lol)
Food Mark (You start craving / can taste whatever your soulmate is eating)
Shadow Mark (Your shadow is actually your soulmate's shadow and reflects whatever they're doing)
"I See You" Marks
You randomly see flashes of whatever your soulmate is doing
The floaters in your vision vaguely tesemble the shapes / surroundings your soulmate can see
Talent Mark (You and your soulmate share talents / abilities) (ex. you can recite 80 digits of pi, so your soulmate can too)
Mirror Mark (The first mirror you see after hitting [x] years old shows you a livestream of your soulmate. Optionally, you feel a tingle on the back of your neck when your soulmate is watching you in the mirror)
Boss Music Mark (You can see your soulmate's basic stats in the corner of your vision at all times) (from tumblr user soulmate-au-bargain-bin)
Criminal Larceny Mark (Looking at an object of small to medium size for prolonged periods of time gives your soulmate the opportunity to "steal" it, teleporting the object to their location) (from tumblr user soulmate-au-bargain-bin)
OMGGG THANK YOU VERY MUCH!
THIS IS A LOT AND I CAN DEFINITELY WORK WITH SOME OF THESE!
THANK YOU THANK YOU HAVE A WONDERFUL AND BLESSED DAY!