honestly? just about anything. i wouldn't buy caps just because i don't wear them, but anything else i'd happily buy. i'm always sad when i go to gaming conventions and i don't have anything frictional-related to wear.
i want to spread the love!
also: while i know the demand isn't as high, i'd really love to see some penumbra merchandise, too.
overall, i'd say that the interest is certainly there, seeing as people are selling plenty of fan-made things on other websites (plush toys, jewelry, shirts etc).