I'm not sure if this is possible with the plug-in architecture or something more fundamental, but I'll lay out the requirements and show how this relates to other topics and previous threads.
Clearspace supports three clear content formats: Discussions, Documents (Online Authored & Artifacts), and Blogs - each having their own characteristics. Great, nothing wrong with this. However, orthogonal to this is a need to 'type' the contents e.g. Issue, Question, Suggestion, Problem, Solution, Change Request,... This can be implemented informally using a title prefix i.e. Question: XYZ. Crudely, you can include this in your search.
The following roles and interactions are envisaged:
Administrators would set-up and administer metadata & content concept types.
Preferable Administrator or Developers would enable custom tabs to be exposed for the concept types, orthogonal to the content formats.
Users would be able to create metadata and find content based on the content metadata
The able to define metadata for the content, including a content type. This would enable more effective searching and open up the possibility to manage the content by additionally grouping content by type i.e. create containers accessible by tabs.
The following high-level capabilities are envisaged:
Admininistation UI to define content concept types and other meta data (similar to people profiles)
Custom Tab Plugin with configuration points to map content concept types created above
Space configuration controls to expose content concept types
UI enhancements and Actions to create and manage metadata instances
Integration of content concept types into search and link selection mechanisms
It is likely that the page layout may need to be reviewed in regard to the number of tabs exposed. This may lead to a wider need to create different views i.e. one for content formats and another for content concept types.
The above is not intended to constrain the solution, but just some thoughts about the possible solution architecture and its inter-dependent components.