Currently, global user pages are not transcluding categories wit the content from the host wiki. Since global user pages do not allow for local categorization, they should at least transclude the categories on the host wiki.
I was thinking we should instead allow the local page to have categories, and still show the global user page if there are just categories locally. I'm not sure how ForeignFileRepo does it yet...
I'm suspicious of the use case here. Categories added to the Meta-Wiki page, just like page links, should not be expected to exist, work or be desirable on the local wiki.
If anything, much the opposite. If more people create global user pages, we'll have a more complete index of categories on Meta-Wiki making it easier to find users.
Having the local wiki display the categories (and link to Meta-Wiki) would imho be useful though.
IIUC... Both would be good.
The simple version is for local categories to work, for people who have a global userpage.
So I can (hypothetically) use my globaluserpage at enwiki, but still be in enwiki:category:User_php-5
More complexly: Above that local category box, there could be a "Meta categories" box - this would contain the best bits from the user-category structures at all our wikis, properly centralized. But that will take a lot longer to discuss/build, as well as needing extensive i18n.
It would also be unlikely to ever completely replace local-categorization needs (eg. Category:Mentor at dewiki, Category:Wikipedia_Teahouse_hosts at enwiki), so the simple/local version will always be needed.
There's a specific request about this here: https://www.mediawiki.org/wiki/Help_talk:Extension:GlobalUserPage#Feedback
Usability issue: it doesn't seem to be possible to include a user page in local categories. After I deleted my enwp userpage, I tried adding it back to a local category (one that would make sense to have here on meta, but it currently only exists on enwp), and after saving the category was present but the rest of the userpage had gone away. After re-deleting my local user page the global userpage reappeared. Additionally, my local user pages aren't being included in categories that exist both on the local wiki and meta.
Example parent categories (and a random subsub-cat or 2):
If Wikidata adds support for Meta-Wiki, maybe it could use Wikidata to figure out whether a Meta-Wiki category has a local version and if so, what it's called.
For example, if https://meta.wikimedia.org/wiki/Category:OTRS_volunteers were linked to https://www.wikidata.org/wiki/Q4656007 it could determine that the French Wikipedia category is at https://fr.wikipedia.org/wiki/Cat%C3%A9gorie:Participant_OTRS but that there's no known Spanish Wikipedia category and react accordingly (put the user in the local category on the French Wikipedia and do nothing on the Spanish Wikipedia).