What new features? Current email infrastructure already implements everything mentioned on this page. There's mention of the "limitations" of IMAP and SMTP but it seems specious. Reading email is a pretty well solved problem now, and I think every popular language has libraries for IMAP and SMTP.
On the topic of IMAP's performance on mobile: there is an installed base of millions who use IMAP for their mail on their phones. There are many, many good email clients for Android (I recommend K-9 Mail, which is FOSS) and they work great. Presumably it is the same for iOS.
If they write a nice HTTP API for reading mail inboxes, that will probably be a nice option for people who for some reason can't or don't want to use IMAP, but there really is nothing here to move mail forward.
Mailpile is a lot more interesting (webmail and privacy features): https://www.mailpile.is/