Such as "Cast Member" and "Former Cast Member." The former should be priority, I think. I know that James, Susan, and a couple others' usernames aren't indicative as to who they are in the podcast, so giving them the custom user rank of "Cast Member" (in place of a poster rank) might give users insight as to who they're chatting with on the forum. Likewise with the "Former Cast Member" user rank, as I'm regularly active on the forum, and should Manda, Zach, Sean, Alex, etc. start lurking and posting about, that'd be something to give them, too.
I was gonna suggest this awhile back, but kept forgetting. A recent post reminded me that it'd be a good idea to implement.