Concerning the question of Sigil and already being feature-rich, or this not becoming part of Sigil itself, I disagree.
Perhaps little I say at the moment will have any effect. I do not ask for what is etymologically termed de-bate, con-vers-ation, discussion, etc. I merely wish to say something, and perhaps from such, the idea might simmer for sometime and someday the concept of what is an EPUB editor may change.
Consider IDEs, the best code editors, or the best text editors. Perhaps Sigil is not such and should not be. Yet much can be learned from such. Or consider the concept of a text document and how various word processors have evolved over the years; even word processors that may not try to do everything, but what is most used by most people while being pleasant to use -- Apple Pages as one example, consider all that such can do.
As for modifying tags, I try to think of what might be the simplest, most elegant, and require the least amount of effort. Such is part of what I think of for each and every feature. For the example of a specific tag, consider how one can easily jump to the CSS definition. Imagine if one could perform any number of operations from merely the cursor point and then some menu command or shortcut: rename class or id, modify (as with what this plugin does), transform to header, div, p, etc., or who knows what. Myself, as I am used to perhaps as the (code) editor with the most features, JetBrains IDEs, much is contextual and much is available from shortcut. I do not ask for as much, yet just as one can select text and apply operations such as styling, operating on tags as such I think would be a big step towards ease of use.
I understand such may take some larger changes. But perhaps someday. Related, I have not tried to notice if Sigil does such, or if such is available on other platforms. Yet on macOS, one can set the menu enabled state. So if for example the cursor was not in a place where a menu item was usable, one could analyze cursor point, possible actions, and then call the App Delegate and change menu state. If such is useful here, maybe. Such could improve usability, such if focus was in the file list with some file or files selected, some menu options such as split at cursor could be disabled.
I was thinking such could be used for perhaps someday more features such as selecting files, and then a menu or shortcut to merge, delete, rename, etc. Much more could be done; much more could be easily accessible.