This idea makes a lot of sense. As a matter of fact, we already had this idea ourselves too. We refer to this feature as "User-defined Regions" (whereas the currently released feature is called "Predefined Regions").
During Sites 9 development, we have conducted several interviews with customers and partners to get an impression of the importance and usability of Regions-related features called "Predefined Regions", "User-defined Regions" and "Stand-alone Regions". From these interviews, it was already clear that "User-defined Regions" add a significant value on top of "Predefined Regions".
However, we also concluded that "User-defined Regions" shine in particular in XPM use cases. This was one of the reasons why we decided to not include it in Sites 9.0 yet (we're working on an overhaul of XPM and didn't want to invest a lot in adding features to the old XPM architecture).
Note: an excellent DXA Module has been created which implements "User-defined Regions" functionality in XPM, using a mix of DXA Regions and "Container Components": the DXA Container Framework.
Regardless, having this submitted as an Idea is a good thing. This allows us to gauge the popularity through votes.
So: your votes please and please also comment on use cases (do you think that "User-defined Regions" has a lot of value if it is a CME-only feature?).