Adding overviews for new maps would need to be done per version and would probably take a bit of time, Bagel is a version ahead of the one being played in UGC already so idk how useful adding an overview for it would be at this stage.
Letting people upload new ones themselves could be useful but could also be abused or just done wrong, so I'm not sure what the most efficient way to get new overviews added would be.
Maybe if generating the overview could be automated that could work but it'd require a computer running TF2 and on some maps the autogen overview could be awful.
Alternatively if there's a standardised way to make them I'm willing to just make a ton from a list.