I have wanted subfolders since I started using Evernote - nearly from it first becoming available. The lack of this functionality has been the top reason for me in the past to discontinue using it and not pay for a subscription for many years. I am back now only because of the redesign that brought it closer to some of today's standards for look feel and user-friendliness but, more importantly, the calendar and task list.
Please, I think we're begging, add a subfolder function. Please, please, please!