These are things that a WikiPortal might do. Wiki as it stands doesn't really give us a good way to prioritize these, but you can help by rearranging the order as you see it. Please also add new stories as they occur to you.
It'll be useless if it doesnt:
Provide wiki participants with a single RecentChanges for all wikis.
I am not sure about this. . .I think if we could get a list of wikis and a short description, with a RecentChanges link next to it, that might work
Let wikis register and deregister themselves as sources of RecentChanges
It'll be lame if it doesn't:
Allow searching/RecentChanges of all sites by category. If a WikiName belongs to several categories it would show up in all of them.
Provide a MyWiki page to give visibility to changes on selected category(s)/site(s) and to keep track of persistent settings as per above.
Permit users to subscribe/filter particular combinations of category/site.