I think it would be great if certain roles could be specialized to operate per category.
For in ticket management system, we have two dimensions:

• the hierarchical organization
• the categories of incidents It would be better if it was possible to give some roles acting on organizational units , and other acting on categories. In a ISO9001 quality management system for example, there are "quality responsables" per organizational units et pilots per processes. A posibility to have this in redmine would be to map organizational units to projects and subprojects, and ISO9001 processes to issue categories - but in current version, pilots will have same permissions on their own process and on all others. Of course, this links this issue to #5358, because it will be simpler if subprojects share their parents categories.