If there is “Privacy Button” in Safari, then why not block it by default? Not needing external uBlock plugin would be a great thing
I would like to see more updates to the Mail.app as I am missing the “Snooze” functionality. Ideally I would like to see some ideas from Hey incorporated into Mail.app (like privacy-related stuff)
I really like new notifications and Mission Control
I would really love to see more updates to the Spotlight like support for more currencies or better integration with stuff - more like in Alfred
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).