-FaceTime with 3+ users
-Facetime that doesn't pause/turn the camera off in full screen apps (sort of mentioned above via PiP)
-(side note - FaceTime should be in landscape by default if between two Macs)
-Better AirPods integration, including clearer task bar indication of connection and widget/some easier way to easily view AirPods battery levels (and better audio quality when using the mic, if possible)
-Fix for live photos - currently, iCloud/Photo Stream syncs just the still image from iPhone and not the video. When importing from iPhone to Photos app over USB, Photos imports the live photo video files SEPARATELY. ridiculous.
-Ability to delete duplicates in Photos.
-Additional free iCloud storage (5GB is pathetic).
I'm sure there is more, but the above irritates me daily.
Also, PLEASE maintain speed and stability, even for older Macs (like my 2013 retina MBP, which still runs like a dream on Sierra).