Similar to post edit history, I would like to suggest history for custom fields. In XF2 all profile fields are custom fields. Threads will have custom fields as well.
There are various benefits for this:
prevent data loss and destruction in case anything goes wrong
easier to revert fields...
It would be extremely useful if we could import tags from vbulletin. For example my site has 35,000 tags which we really need to keep. I'm sure there are many like me.