Other things you notably can't separate on a single wiki: search defaults, main page, sitenotice.

(In reply to comment #1)

Other things you notably can't separate on a single wiki: search defaults, main
page, sitenotice.

Hmmmm. Right.

What the main page is considered is most relevant to the logo and the sidebar, I think. So I guess you could implement the ability to have per-namespace sidebars and then also implement the ability to have per-namespace logos/logo destinations. Maybe.

Search defaults... Special:Search is its own independent namespace. Maybe you'd want search namespace groupings or something (that are configurable more easily than the current groupings)? Can you split this out into a separate bug, please?

The sitenotice can probably most easily be changed per-namespace by modifying the sitenotice extension's JavaScript to check for $wgNamespace or whatever. That's probably better than doing it server-side (for caching reasons).

Also: https://meta.wikimedia.org/wiki/Grants:IdeaLab/A_place_to_work_together (for private content and ACL).

(In reply to Nemo from comment #1)

Other things you notably can't separate on a single wiki: search defaults,
main page, sitenotice.

Would need separate bugs.