So, here are some suggestions:
- In the usercp to show extra info for the favorite stuff. Eg for the threads to display posts, views, last post, like in forumdisplay. For messages it could display how many replies were made after it, in which thread it belongs, the date it was posted etc. For users it could display posts, last visit, online/offline status.

-Show something to an item that is already in the favorites list so that the user doesn't try to add it again. If it's a thread, change the title in the item inside thread tools to display "Remove thread from favorites" instead off add. Same thing could be done with posts and users
This for now, I'll edit this post (if nobody has replied) or post again if I have more ideas