It seems to me that there are a lot of tags with no wikis. I think that tag wikis are a nice feature and it'd be great to have more of them.
I propose that when there is no tag wiki at all we allow even users who are not Top 20 Answerers and who do not have the bronze badge in the tag to create a new tag wiki providing they have enough reputation. It seems to me that the reputation restriction is sufficient to deter vandalism. We could even raise it to say 5,000. This should allow for the creation of a lot more tag wikis.
Also, when you get a new tag Badge for a tag that has no wiki, the message informing you of your new badge should also encourage you to create a wiki for the tag if there isn't one already.