Batch renames meaning creating a new tag that doesn't exist and deletions being removing the tag entirely.
Also, if it's not too much trouble, I think that the tag subpages should be improved. The tag wiki should be its own page and the landing page when you click on a tag should have the top 5 questions, 5 newest questions, and basic statistics on it. Or something. I'm not a fan of going right to the page with the latest question with that tag since usually that's not what I want - it's usually the info page, stats page, featured page, or hot page.
Oh, and when I'm a roll...how about a way to discuss the tag on the tag? Where people can talk about merging, renaming, deleting, proper use, etc?
An example of where these features would be useful is dealing with the "requirements" tags. I want to easily propose that
software-requirements be merged into
requirements-engineering as all questions in the first 3 and it appears some or most of the questions in the 4th are all about requirements engineering. Since StackOverflow is more programming related than software engineering, it would be best to use a broader term here - there's no need for the granularity in topics such as requirements engineering that you would need on a stack exchange dedicated to software engineering. And since this is a huge undertaking (relatively speaking - several tags are being affected), it would be nice to have a place to discuss this other than on meta so that people who follow or participate heavily in these tags can find out that "hey, people want to do things to these tags".