Something I wanted to make was an "organization" extension, similar to a page on Facebook for example. You could create a public profile, add managers, and then you can interact using that organization just like you would with a user. A switcher somewhere would allow you to switch between your user or any of the orgs you are a member of. Optionally you could see both the user who made the post and the organization as which they acted. It's a bit similar in the idea.
Having posts with multiple authors could definitely be added. A new sort of relationship would have to be added, but that doesn't look too complicated. The most complicated part is probably all of the alerts and prompts and confirmations needed to "invite" a user to share a post, and where they can accept to be featured as co-authors. If it's just a permission where anyone in some group can co-credit anyone else, it's probably not too much work. Even easier if the co-authors are purely visual and don't actually need edit permission themselves.
That being said I don't see this being super useful on a forum. Maybe only for the first post or discussions of announcement discussions. Maybe for a forum where user share their creations which might be co-authored. I don't think this would make much sense in replies.
