Personally, I think that the current tag cleanup process used for a long time now seems perfect for non-controversial removals of little-used or newly-created tags that should be removed preemptively to avoid future problems. However, it was not so useful for tags with lots of questions tagged and whose removals would be very controversial. slhck and I came up with a new procedure for sorting out tags in the former category and in the latter category.
My (counter-)proposal is this, which makes big modifications to the existing tag cleanup based on ideas established in the question:
Tags whose removal wouldn't be debated for sure will go through as before. Tags whose removal would be debated or tags in which the one who finds the tag doesn't know what would happen would be posted as a separate question, and then if the consensus is to clean up, would be added to the tag cleanup.
What should be done now:
There are now two lists in the main answer ("uncovered in the audit") to the existing tag cleanup: one with little-used or newly-created tags whose removal would not be very controversial, and one with tags that have lots of questions tagged each whose removal would be more controversial and debated. Each tag from the latter list should be subject to the procedure currently outlined in the question for community consensus, and if the decision was to clean up, it should remain in the list, and if the decision was not to clean up, it should be removed from the list.
What should be done in the future:
If you think the removal of a tag would not be very controversial (rule of thumb: less than 200 questions tagged (though that's a ridiculously simple heuristic that may need to be changed)), it should be added directly to the answer in the first list. If you think that the removal of a tag would be potentially controversial (rule of thumb: 200 or more questions tagged) or if you're not sure, it should be posted here as a question following the procedure outlined here. Again, if it meets community consensus, it will be added to the second list, and if not, will not be added at that moment.
Other than this, the tag cleanup will work as it has always been, except that moderators should add the same number of tags from each list into the question (I think two each is a good number).
Personally, I feel like the existing procedure favored the non-controversial tags, while this new one favors the controversial tags. My idea takes ideas from both and creates a very good procedure that works for both kinds of tags.
One last question: What will we do to prevent users from adding their own favorite tags to the post? We'll say that all edits that add tags to the second list must link to the decision to clean up in the edit summary, and all edits that add tags to the first list must have less questions tagged. If the respective criteria aren't met, the edit will be rolled back.
What do you think?