Add some way (e.g. a tag cloud) to see all tags used within the current list and click on them to filter
If you're working on a big outline with lots of deeply embedded item tags, it'd be really useful to be able to see all those tags in a list (e.g. as part of the 'list information' pop-up accessed via the 'i' button) or a tag cloud.
Clicking on a tag should then filter the outline accordingly.
You can 'sort of' do this by just typing '#' into the filter field, but that immediately lists all tags used throughout your account, whereas what you want is tags used only in the current outline.