Oh well, when I started to read your question, I felt as if I'm reading a project description on Elance or Freelancer.com website! ;-) I've not used slack.com but my first impression after reading your question is that the implementation of final solution requires some coding beyond theme for bbpress. Not sure whether this sort of question is withing the purview of StackExchange rules or otherwise; may the moderators take wise decision about it.
Moving on, I believe you require (at least) to implement following capabilities in order to accomplish this project...
- Front-end Editing
- Upload Files (Images or anything) from Front-end
- Discussion Board
- Rating system or something that will mark a thread as "finalized".
Please correct me if I've missed anything here or walking on a entirely wrong path.
Point 1 can be accomplished using some of the existing plugins. I'd make each question as POST and I'd use meta tags to attach images or files uploaded to attach/associate with a specific Post (later, I can simply get the file names and links from the Post meta to show in my theme file). Wordpress Comment system will accomplish the Discussion Board whilst the ajax with Post-Meta will accomplish Point 4 above.
Showing all the attachments of the Discussion Thread in a specific sequence is a matter of writing code in Theme file and enlisting attachments.
I know this answer is very gross outline and requires some more thinking. But if you've noticed, this solution does not require bbpress.
Looking forward to hear from other members as well; I'm sure this discussion may lead us to a good foundation for the project you're discussing.