Thank you for contacting us, MrX.  If I understand you correctly, we already have this feature.

While the Content tab is displayed, click the "+" at the right edge of the window, near the top.  There are also keyboard shortcuts in the Selection menu, available whenever an item is selected (so we know where you want the new item.)

I understand that some might expect this to be in the File menu, because of New…, but we like to think of File as relating to the hard disk.  Creating a new folder in a bookmarks management app is more analogous to, I think, creating a new text box or shape in a drawing app, or adding a new page in a word processing app.