It would also be very, very good to remember what each member has read and use that info in deciding what to present as a 'new' item on each forum thread.
Decoupling the presentation of threads from forums (soft or hard links, in file system parlance) would be a help, along with a 'forum' that gathers the new threads from all forums. Giving the thread creator a list of topical 'forums' in which the thread might also appear would help. And giving users the ability to create their own 'forums' that gather threads of interest would also be nice.
Of course, allowing groups and forums to be hierarchical would be nice, too, as long as we are hanging out wishlists.
The difficulty of all of this depends on the clarity and form of the 'data model' on which all of this is based. Refactoring is always easiest when it's done as the code is designed.