When I am working with issues, I have too many limitations from project and WF administration. I cannot create any additional WF issue status to have ability to group issues in my way.
So I think that here is very simple solution:
Allow users to create any number of tags to any issue.

Format of tag can be very simple:
Key (text field) and Value (text area).
And user can filter issues by entering pair of key and value.

Good thing would be predefinded profiles of tags:
User can create it`s own predefined tag keys and unite tag keys in profiles/groups.