It would be good if there was the ability to set tags to be hidden from the tag list (below the folder list).

For example, I'm currently selling a property and have a tag set up for such emails. In a few years time, long after this property is sold, this tag will just be clutter on the main interface, but I don't want to remove the tag from the emails as I might need to refer back to them at some point.

I imagine this would also be useful for people who use tags for projects. The projects would end, new ones would start and the list of tags would keep getting longer. They will still want the emails tagged for the projects in case they need to refer back to them in the future. If they could hide some tags, they could just keep the current project tags visible and the rest hidden.

This is something that is available in Gmail. In their settings for labels there is the ability to hide/show each label.