Now that ZFS support is wrapping up I would love to see some focus return to the star feature of UNRAID, the unRAID array.
First and foremost I would like to see formalization of the array as just a type of pool concept, both in documentation and nomenclature in the GUI. Having to explain the difference between the array and other pool types gets old after a while.
It would also be cool to see the unRAID type pool be augmented with some more advanced features. It would be really nice to be able to replace or re-add a failed device (or expand the pool) without having to take it offline (hardware permitting of course).
Adding an option for a read cache would be really cool, either a write-through block cache or some way to intelligently promote files to some sort of read-only overlayfs.
My biggest wish would be to see some attention given to the file integrity properties of the unRAID type pool. Things like ensuring that fs read errors are properly propagated through fuse layers and samba (pretty sure there was an issue raised about this at some point).
The biggest longshot, but coolest feature would be to see the BTRFS check summing feature (and ZFS I guess) integrated with parity checking similar to what Synology hybrid raid achieves.