Steve Wiseman

I don't know if you are aware but you can reuse content between Categories using Paligo. It's also here in the Zendesk marketplace. 

Paligo allows all the functionality mentioned in this long request as well as other pieces of functionality that authors may need such as versioning and collaboration. 

Zendesk is superb as a support platform and Guide provides excellent functionality for showing content, personalization, ticket deflection, widgets and the like. Paligo simply adds an enterprise level of authoring on top of Guide. The combination maybe of interest. 

[Yes I am connected to Paligo but I still think this information is relevant here]