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My main site is Ask Ubuntu. I'm not sure how many tag wikis I've edited, but probably not many more than it took to get the shiny silver Research Assistant badge for editing 50 tag wikis that's also been earned by twelve other users on our site.
Good tag wikis are important and I know I should be writing them, but inevitably I put it off in favour of some other work... a badge would make the difference between getting round to it and not getting round to it.
I know writing and improving tag wikis is a slog and most people won't do it even though there are badges, but that's ok. A site only needs a few
badge-pursuers enthusiastic wiki-makers to make a big difference.
So I think there should be a gold badge for making, say, 200 tag wiki edits. It could be called Doctorate (because of the research required), for example.