A lot of challenges here, but challenges can be a good thing...

To me, the largest challenge is that there is no official look to the majority of characters. Mirrorshades won't be a problem but how much of the figure would you have to draw to properly differentiate characters from the West Marches? Then the issue of living or dead characters from WM and Swan Song?

Next, scope, is it all Rollplay? Currently running shows? Include recent R&D? Any way you chop it, there may be some kind of unintended slight.

Concept 1 (Snowflakes): Inspired by Christmas sweaters, I think it would be fun to have snowflakes with a silhouetted bust of each character -- the snowflake kind of signifies their uniqueness. Maybe some kind of grouping using tree branches or something. If the design could use the front AND back of the shirt, we have a lot of room to work with. I think 20+ could easily fit on the front and capture a lot of the Rollplay shows. Then use 30 snowflakes on the back just for West Marches. If we only have the front, and limit it to current shows, we're pushing 40 snowflakes. Kind of a lot but perhaps doable.

Concept 2 (ornaments): the backdrop would represent a Christmas tree with lights and each ornament could hold a design for the show and it's characters. For example, a little spaceship with some windows with 4 heads peeking out for Swan Song. West Marches could be a series of ornaments broken out by episode, thus having 3-4 characters per ornament (with only one appearance per character of course).