There are hundreds of different soulmate AUs out there, but I think my personal favorite is the one where when soulmates touch, color appears on the skin where contact was made.

Just the imagery that I have of this is just so beautiful to me. Imagine drawing little pictures and patterns on each other's skin. Figuring out how to make the colors change to create something wonderful. Watching the color on your skin slowly fade from the last time you two touched.

And of course there's the potential angst material that could possibly come with it. Someone who doesn't want to find their soulmate always wearing long sleeves and gloves and high collars and scarfs in order to prevent skin-to-skin contact. Someone discovering who their soulmate is after being slapped by them, and the other person brushing off that splash of red on their hand from just being the burn from slapping them.

But also imagine that after soulmates discovering each other, then just being so affectionate with each other to the point where there's almost never a time when they don't have color on them. Lines and patterns on their arms from hugging, a big blotch of color on a shoulder from where the other rested their head, little doodles on each others's ankles, colorful outlines of lips on their cheeks, etc.

I just... really love this concept.