Oooo i like RazorLeafs idea but generally, yeah, heros on the shirts with a quote.
Maybe a Team Alric/Team Eridan shirt? Or one with the two characters glaring at each other while they arm wrestle? Itd be cool to see some tshirts like that with other characters interacting with each other in various ways, maybe denas sharing a mug of beer with Rurin?
I know i said "heros and a quote" but in this instance text itself is a bonus because the image would be the main focus. (Though for Denas and Rurin for example you could have at the bottom of the image: "It's Party Time!")
Id also like to see something like this on a shirt:
Minus the games logo and an original battle scene for preference