I'd like to second this - The example given above is not only desirable for performance and media reasons.
While we can already locate shares on specific media, I would like to be able to demarcate a smaller array with it's own parity disks for a shorter rebuild for more important data.
(Array 1) - 3 x 10TB HDD + 1 x 10TB parity for mass media storage
(Array 2) - 1 x 4TB + 1 x 4TB parity for NextCloud storage with multiple users
Sticking with the given example, with 4 x 10TB and 2 x 4TB disks in a single unraid - even if I explicitly locate my media and user shares - if I lose a 4TB disk, I'm waiting for 6 x 4TB + 4 x 6TB of parity processing before my user data can be considered "safe".