There are many fine suggestions for radically improving the feature set for tags (eg Suggestions for Enhancing Task Tagging and Time Tracking Efficiency )
However, I think a more fundamental criticism of tags at present is they aren’t accessible.
Currently tags are administered within a modal in a list view that doesn’t have a filter or search. That’s not very assistive as per The Chartability Workbook . The same goes for Custom fields, to be honest.
Before focusing on deep new features I would ask for feedback on Toggl’s plans for making navigable interfaces for these properties.