Aside, this bit that Neil and the team are working on is something I think is the 'next big thing': https://www.ietf.org/archive/id/draft-ietf-jmap-emailpush-01...
The next, next big thing would be the Chatmail relays[1] supporting JMAP based servers (right now it's Dovecot) and this new targeted push extension for faster notifications without battery drain on mobile. I can see how the Fastmail mobile client will benefit from this RFC as well (it's already incredibly battery efficient, thanks to the team).