I have joined materia slots on my shoulder. I have never regretted it for a second haha.
I want to get more ink, but that was the only thing I was ever TOTALLY sure I wanted and I can't think of anything else. I was thinking of the Sign of Chaos, but I dunno... I'd feel like a poser since I don't play the tabletop Warhammer game.
Also, I've got no one to impress with tattoos, and (kind of like smoking), very few people will consider NOT having a tattoo a deal-breaker, but plenty of people will. See Kristian Fischer's post on the first page.