As a result of http://meta.stackexchange.com/questions/128315/the-great-stack-overflow-tag-question-cleanup-of-2012, a number of ambiguous tags are in the process of being cleaned up.
However, as they get cleaned up, and reputation requirements (currently 1500 on Stack Overflow) for new tags are put back into place, I've noticed that there are still people that will use those tags and resurrect them (I go over the eliminated list regularly).
That said, the feature request is require that if tag has been created before, but then removed from all questions, then the amount of reputation required to bring the tag back should be raised (or perhaps only allow moderators to do so). By how much is left to you, but it should be very high, IMO.
Also, the following conditions should be satisfied:
- The tag is of a certain age. This will prevent someone who creates a new tag on a question and then removes it shortly after from triggering this restriction
- The tag has been applied to a certain number of questions at one point. This also helps with the situation above, as well as for tags applied to a smaller amount of questions.
The main goal here is to help massive retagging efforts to tags which don't have synonyms applied (because there it's not a synonym but just vague) from going to waste; there's no point if the tags have been removed if they're just going to sprout up again in places we don't see/know about.