Tooltips and fixed dropdowns pinned to the position of page elements are already fairly trivial to do... so are modals... so are dialog boxes... so are every other thing that needs to "Pop Up" on your webpage. I predict a very fast death spiral for this tag, in which 99% of the uses will be obnoxious.

You know how, uh, ad blockers used to be called "popup blockers"?

There's an order, a z-indexing that creates a syntax and a hierarchy in everything you build. Sometimes you have to jump it with absolute positioning. Which already exists for this exact purpose.

CSS itself has suffered from this same line of thinking, like, "what if something has to override a class and override a style, should we call it !important? Yeah!"