Folders - Be able to create subfolders.
For example folder with surname Jordan. Subfolder for a particular name or record number. It might be helpful to organize source objects that are not yet attached to a person.
The Source Box was originally envisaged as being a temporary home for sources that could not be attached to an ID for the "time being". I still use mine in that way.
However, many users now use it to store records on a more permanent basis. Maybe, in time, the developers will appreciate this alternative use and introduce enhancements - such as what you are suggesting - to suit the needs of what is possibly now the vast majority of Family Tree users.
I am just another 'lowly' User/Patron ...
And, one who has a "Source Box", that is very extensive ( ie. Almost 32,000 ).
That is a GREAT suggested enhancement.
We have recently been advised that 'FamilySearch' is, FINALLY, currently working on several enhancements to the "Source Box", which by some is considered long overdue.
And, later, in that same post, a list of what is being worked on; and, a request for any OTHER suggested enhancements.
Rather than getting lost in the System ...
I will "mention", the Author of the aforementioned 'Comments', who is, BOTH, from 'FamilySearch'; and, working directly on the enhancements to the "Source Box", just so as (hopefully) he is aware your suggested enhancement.
I hope this helps.
Attention: @Bryce Roper (FYI)