Limetech have already stated that they intend to make the current Unraid array effectively a pool type, and that in the future a feature will be to have any mix of pool types you want. The difference being that you will then be able to have BTRFS, ZFS and Unraid type pools in whatever combination of primary and secondary storage that you want. Also note that Unraid type arrays/pools can already have individual disks in BTRFS or ZFS formats without losing the current easy expansion capability of the Unraid array.
Do not forget that you keep the flexibility you mention even in the extremely unlikely event you mention of XFS being removed from Linux (which I would think would be at least 10 years off if it ever happens at all). After all it wil be about 5 years between the first announcement that ReiserFS would be deprecated and the date that it gets removed from Linux kernels. That is despite the fact that ReiserFS has severe technical limitations (i.e. it cannot support modern large drives) that are not the case with XFS.