Many CMS’s has the opportunity to tag across content types, in order to find similar/related content. I really think this could be a nice feature to add.
Also maybe since the tag field is localizable, it could be nice that you could have different language versions of tags, so the editor can tag in their own language.
BTW, really great work with this. We are about to implement it and so far everybody is very impressed, so keep up the good work.