what if Rin's magical tattoos bloom or wither due to the weather or his mood?
Aaaah weather sounds cool but since he has a nature familiar, it doesn’t seem as helpful. Definitely a no on the mood one, because can you imagine his mortification about having his moods seen by others the way Nagisa’s are? He won’t ever willingly let anyone give him that kind of tattoo.
Unless the tattoo was a curse...?
My first thought about the magical tattoos was that they were more or less a power gauge that lets Nagisa know how much of Rin’s magic is still available. The rose wilts when he uses too much, and Rin gave Nagisa permission to stop him when he sees it. It’s one of those secrets he shares when they finally start working together becoming a real family.