I sometimes find myself considering IList<ContentReference> just to force users to create the block first. But that is usually too restrictive -- you can't use personalization, have to render the contents yourself, etc.
A couple additional ideas to enhance the UX:
I don't think using IList in this case is relevant for my request?
However your additional points sounds nice.
I love the "Create a new block" feature. It's a quick shortcut for the editors to easily create a new block without first finding the right folder, create the block. Go back to your first content (yes you can use the back-function in the browser... But still...) just to drag it into the correct Content Area.
I would like that the Editor should be able to Click on "Create a new block" and get a folder picker. This is where the block will be created instead of in the Content Asset Folder.
This would keep a nice shortcut, but with the possibility to make the block shared to other content without the editor moving the block around from the Content Asset Folders.