I really hope Fastmail implements the JMAP spec for calendars and contacts soon. They’ve had the mail part of the spec implemented for a while, but it still requires CardDAV/CalDAV for contacts and calendar access.
What good is a protocol like JMAP as long as common clients like Thunderbird, K-9 Mail, the iPhone email client and others don't support it? Without some concerted effort it will never take off. Then there is the question of what problem it solves that isn't already solved by existing solutions.