These are just a few things I thought might be handy for bbPress.
1. Invitations - would allow users to invite other users to the forum and maybe give them credit of some sort for the referral.
2. Cash - this is one of the really popular phpBB plugins, it gives users cash for posting, and sending messages, etc, and sets up the facility for other plugins to use the cash for their own various means.
3. Original Poster - perhaps next to the most recent poster or under the topic title, this would show the original author of the post on the forum view, instead of having to view the topic to find out. Just one thing that sort of bothered me, as sometimes I like to know who started what.
4. Subforums - I saw this somwhere, I think, but subforums would be a wonderful addition.
5. WYSIWYG post editor - I assume this is already in the works somewhere, but for folks who aren't good with tags and what not, it's really not userfriendly to post in bbPress. Perhaps just a copy of Wordpress's editor.
6. Custom Profile Fields - I think a plugin that allows admins to change the profile fields that are currently shown, add more, remove some, etc, would be really nice.
So...those were just some ideas I had, maybe they are not feasible or easy, but hey, at least maybe I got the gears rolling.