This is really cool! I'm glad you guys are doing this.
With respect to designs, my wife and I would both prefer more art that's designed for t-shirts... stuff like the iconics can stand alone in a book, but on a t-shirt it makes for some awkward shirt designs that we're not really into. (Unless it's in remembrance, you don't see a lot of shirts with a single depiction of a person on it, even if that person happens to be a fictional character. It's just a little odd.)
For an example of what works really well, the various Pathfinder ales are awesome. (The alcohol themes do rule them out for us, sadly.) If we could see more quirky stuff like that we'd be spending some money.
Could be stuff like travel shirts (visit the splendors of glorious Rahadoum!) or fun faux educational or team shirts (Arcanamirium Siege Club?) or whatever--there's a lot of tropes that would translate well!