For example better integration with reminders, or using special IMAP folder for that.
In addition:
- Support for JMAP, Apple has long history of being one of the companies that forces good ideas (invertible universal connector, abolishment of Flash in web)
- Allowlisting senders
- Stream with all newsletters instead of “mail by mail” (when was the last time you needed to respond to the newsletter)
- Support for JSON-LD with GMail-like actions (one click for confirm registration, adding event to calendar, etc. even without opening the messages)
- Automatic blocking of tracing pixels
- Message threads merging and renaming
From nice additional things it would be being able to annotate email messages with own comments (it could be implemented for example on top of IMAP notes).