Assuming we want to encourage tag wikis to focus on site-specific information about how the tag is used — following the model of the much-loved scala wiki — perhaps Research Assistant, Cataloguer or Archivist would be appropriate.
This badge could be a very good thing because it might break the chicken-egg cycle of "I don't bother reading tag wikis because there's nothing good in them" and "I'm not going to waste my time writing a tag wiki that will never get read."
That said, I'm still not entirely sure what need tag wikis are supposed to fill. They don't seem to be targeted at newbies: for one thing, the sites are about questions and answers, and tag wikis are obscure and hard to access by comparison; and for another, if I've never used (e.g.) MongoDB, what could a description of how mongodb is used on SO possibly offer me? As far as I can tell, the answer is nothing, especially if we're not going to provide general encyclopedia entries.
So we're down to tag wikis being used by people who are already at least somewhat active in the tag. Curiously, the more active a user is in a tag, the less that user is likely to benefit from a "best posts" list (as in the aforementioned scala wiki). The questions page can be sorted by votes, activity and post time, so the wiki doesn't need to fill that role, and the rep leagues already highlight the top users, so that's out too.
I wish I could end this mini-essay by saying that I've figured out the perfect job for tag wikis to do, but... I haven't. It might be interesting to see a "users who have this tag as a favorite also like X" section, or a "top answerers in this tag also answered questions about Y" list, but those would be sidebars at best.