If you don't mind people horking weird ideas all over the place —and please feel free to ignore me if you do; I get too invested in things—

One of the characters could have a dangerous or thrilling side job that comes back to haunt them and drives the plot. The protagonist (or tattoo artist) could be a detective working on a dangerous case (or a lawyer, or [retired?] assassin, or active soldier, or government official, or psychic medium, or—). Or both of them could have pasts that would otherwise put them at odds or pit them against each other if not for that positive encounter in the tattoo parlor, and their outside lives start to clash even as they personally become closer. That way the relationship and its ups and downs could echo or run counter to whatever perilous event is waiting to happen and increase the tension and stakes.

skjdlakjldkj sorry, I don't know. But you already have two interesting characters with a compelling dynamic, and that's amazing.