Thank you for your input! The access control question is a good one, it hasn't been discussed yet, but I will keep that in mind.
So as I understand it, Namespaces are quite useless (or not very practical) in a wiki related to the wiki content, they should be used only for structural things not relating to the content of the wiki (like Discussion pages, Special pages, Special pages for extensions, etc.)
The project I'm thinking of would be a cooperation of two (or more) cities. They could have their own pages in the wiki regarding their own topics. But cities from the same region share the same regional laws and therefore can cooperatate in putting knowledge in the wiki that is of common interest for them.
Consider a page type "procedures" with descriptions on how to carry out public services as manual for public servants [[Category:Procedure]] with the attribute [[scope::CityA]] or [[scope::Region]]. This sounds quite straightforward to me.
But I also could define the pages being of a different type:
[[Category:Procedure_CityA]], [[Category:Procedure_Region]]
I think the choice between the two depends mostly on the question, whether or not the page types share the same structure (attributes, templates, etc.) which would lead to the first approach or if the pages have to be independent from each other (which would be easier in the second approach).