I haven't seen anywhere a list of used tags, i.e a tag cloud. That would be useful to sort or select just after used tags. That way you can see in what tag you have most todos, you can easlity select & sort after context (eg @work, @call etc) . this is a feature i've seen in for example RTM that is very useful.
such feature is already available. With Todoist Premium, you can see an @ symbol below the search box. Clicking on it will show all labels with the amounts of tasks they've been used on and you can edit their names and change their colors from this view.