Add a setting for Minimum Free Space on a per drive basis.
I use a very mixed set of drives (HDDs & SSDs, wildly different capacities) in my array, and for speed and drive age reasons use the Fill-up allocation method. I've got all my drives formatted in ZFS. ZFS is known to perform badly when drives are almost full, so I'd like to set a minimum free space on each drive that is appropriate for it's size.
Another option is to allow a percentage in the existing Share minimum free space setting, so each drive will leave some percentage of it's capacity free. I noticed there's already some sort of support for this, added in 6.12 I think, but it seems to just pick a static number on save.