Feel free to ignore this, or weigh in. I'm just thinking out loud, so to speak.
Checked out what the official Green Day store had in the ways of t-shirt designs, just to see what was there. Nothin' but boring 21st Century Breakdown stuff and some old designs from Dookie and Nimrod. Excellent. Looked through part of my photobucket, too.
No ideas forming quite yet, other than doing something geographical, like a signpost with directionals for places they mention in their songs like Christie Road, East 12th Street, Stuart & The Avenue, that sort of thing, with the actual distances from, say, their recording studio on The Embarcadero. Or a map of the East Bay with those places noted. Or one of my Agitprop-style posters. Kinda meh on that idea. Unless I did four of them, but that would mean I'd have to include Jason White, and I don't consider him a member of Green Day, no matter what Billie Joe says. Another idea I had was to look through all my 1970's punk records and make like an album cover in that style of them, but use a picture of them that shows how ridiculously un-punk they are for it.
Still thinking.