Implement a full-featured Calendar in Mailspring, with sync support for all (most?) major calendar providers.
This, along with Bundle: Contacts, will make Mailspring a fully-fledged PIM.
Read/write support of calendars in CalDav, iCal/Apple, Outlook, Google.
Subscription support for WebDAV.
View, create, edit, delete events.
Freely associate email accounts with calendars.
Proper RSVP support.
Allow issuing invites.
Appointment reminders via notification.
Task list support (CalDAV, Google)