Improve address book and contact management support.
This, along with Bundle: Calendar, will make Mailspring a fully-fledged PIM.
Read/write support for CardDav, Outlook, and Google. (Also vCard?)
Allow freely associating email accounts with address books.
Create, edit, categorize, delete contacts in address books.
Customize collected address storage behavior.
Pull profile photos from address books, Gravatar, and other sources.
Mail merge, potentially with address book groups, and definitely with CSV files.
(Proposed) Allow caching some Contact Pane info in address book (also reduces API calls!)