Why not just get a tattoo of the Celtics jersey? And by that, I mean tattoo your entire upper body in the scheme of the Celtics NBA jersey, and have the number and player name of your choice in white across your back (or green, if you're going with the home jersey). Think about it, CELTICS would be tattooed across your chest. Nobody would ever be able to question your fanhood with that tattoo.
But in all seriousness, I can't give you a constructive idea. I've been contemplating getting a tattoo for over 5 years now (non sports-related), but I haven't been able to think of an original and meaningful design to fit my liking yet.
Haha I have joked about doing that with my friends. Like I said, I am a diehard fan, so my tattoos all mean something to me, something I am passionate about. I am in the same situation as you, I can't think of what I want to get, so I am just taking my time.