Instead of creating contacts manually one by one, it would be great if Missive had a section to display a list of any e-mail that are in the inbox or sent folders (basically all previous From: or To: e-mails), something like a label "Uncategorized" or "To be sorted".
It would be much quicker to go into this section and go through each e-mail in a list in order to add them, discard them or eventually even blacklist/report them as spam.
Currently, the quickest way to add a contact is to use the sidebar while browsing a conversation, it's a nifty feature but if we want to build our entire contacts book this will be very tedious so there's still room for improvement.
Regarding blacklisting contacts or reporting them as spam, we could also have a label "Blacklisted" so that we can go back into this should we need it.
It could make sense to have a "Discarded" label, which are basically contacts that do not need to be sorted or added into a contact book (i.e: newsletter e-mails).
Finally, a nifty feature with this would be to automatically detect all e-mails from the same domain so when you add firstname.lastname@example.org in your contact book, Missive could display a prompt along the lines of "7 contacts were detected with e-mails ending with mycompany.com, do you want to add them in the My Company profile?". All contact detected automatically would be removed from the label "To be sorted".