Well maybe they'll improve it window/display management.
I'd kind of like to see an improvement over the current side-by-side full screen mode. Perhaps something that adapts to screen orientation/size (i.e. ultra wides would probably benefit from 3 or 4 side by side, a rotated display would benefit from vertical split, etc.
But, as I'm right now dealing with it: I'd love for the built-in software RAID to get first-class RAID10 support. It can already do it (make two RAID1 sets, then RAID0 over them) but its clunky, and Disk Utility almost always says one of the members is "missing". It'd also be nice if it was smarter about rebuild/repair options. For some reason it thinks of the disks in one of the RAID1 sets failed, but then also decided to rebuild that disk (without me replacing it), so I added an extra disk (same size) I had to the drive array and tried to add it to that RAID1 set, so it would repair onto the brand new disk rather than trying to repair back onto the existing possibly damaged disk - and it said it can't, because it's "repairing"..