We use our forums for a Role-Playing game, where a user may have more than one Login (for more than one character in the game). However, it's annoying to log out, then log back in as anohter user to switch characters.
It would be nice if you could link multiple IDs to one user, so that if I was logged in as User1, and wished to post as User2, I could do so without having to log out and back in, then back out and in again when I wanted to post using my primary User ID.
(This feature is sometimes known as "proxying" in Email systems, where a user is given permission to read and send email from someone's inbox, such as a secretary from thier boss' email.)
Security would be the main concern, but that shoul dbe fairly simple to set up by needing permission and passwords from both IDs.