Let's say, I have two tags that I inadvertently named differently, yet they pretty much mean the same thing. I'd love a way to merge multiple tags together!
This could happen within the search overlay by searching for multiple tags and clicking on an option to merge the two tags. A similar function, splitting one rather general tag into two more specific tags, can be accomplished through multi-selecting notes and adding a tag in bulk (like "Edit properties" in Notion databases).
Looking at your roadmap, this could something you consider adding as you are working on the multi-tag filter :)