I'm glad to hear that a new forums module is on the list for SF 5.0. One feature that seems to only exist in very custom forums that has a pair of interesting use cases would be the ability to post to a forum either as another user or alias. An active (and popular) example would be the World of Warcraft forums where a user can post as any of their in game characters.
The two cases I see this being useful for are:
1. Game Based Forums - Essentially the World of Warcraft forum example, where a user has a single account but multiple "personalities" (without actually being crazy).
2. Message Delegation - In our particular case a medical school, but it can easily apply to any large enterprise. Similar to e-mails sent "on behalf of" someone else, the ability to delegate message duties to an administrative assistant or a centralized department contact.
I have yet to find any commercially available forums that implement this feature