Integrate on&offline accounts into one -> see and search imap+mbox at the same time
With limited (online) storage for emails, one is forced to store older emails locally. Outlook does a great job on this: auto-archive older emails to the exact same folderstructure as the one being used. Apple mail sucks at doing this, needing to create manual local folder, and setting up rules.
Airmail - almost there. Just make it possible to integrate a sort of 'on my mac' kind of setup within a imap-view