Mail currently does not allow nested folders to be "closed" so that the while folder hierarchy is not visible. This is very inefficient for the user's workflow having to scroll down through 100 of subfolders vertically, some of which I actively do not want to see, at that time (which is why they are nested) and is frankly crazy for Apple not to implement on the iPhone one of the very features which they invented to make the Mac OS the joy it was.
I would buy any app which restored this function to email folders and subfolders on the iPhone, for this feature alone.
It makes complete sense to me to have at least the same control over folders on the iPhone as I do on my Mac, since I will be carrying it with me at all times, not just in my office.
This function might behave rather like this:
Save an email to a folder/select folder/subfolder(s)/release email and it drops into that folder without seeing everything else. The folders could 'pop' open if the tap is held. The user might want to leave a path of folders open while a particular exercise of saving emails to particular folders.
OR- another series of operations which delivered a similar functionality.
As I say, I would pay for an app which delivered a smooth version of this funtionality alone.