I think for the next expansion it’s definitely time for them to consider spec redesigns again. We’ve essentially been playing with retrofitted legion specs for several years now and it’s just getting stale.
Blizzard constantly defends their borrowed power systems as ways to “change up gameplay” but they don’t do a great job at this as outside a few rare exceptions (shadow priests etc) all that changes is ability priority.
Going further, by being tied down by legion class design they are hamstrung to design anything new and exciting.