Simplify default folders
The Inbox/Outbox/Sent distinction feels a bit old-timey and doesn't make much sense with a conversations-based UI, since your own messages are part of conversations as well. It's conceptually very weird that the Inbox contains all conversations except emails you didn't get a response for.
I'd be interested in exploring combining these folders into a single folder with all conversations (except ones which have been explicitly archived/deleted/etc).
This unified conversations folder could either still be named Inbox, or have a different name, e.g. "Conversations", "Home", or "Mail".