I think Art of War is fine, its just bland as far as proc's go. I don't think AoW would need to be removed for Divine Purpose to become baseline though. They could both easily exist together.
Maybe its just me, but it feels like baseline Divine Purpose is what is missing from Ret's rotation at the moment. If you take it as a talent it feels too RNG and doesn't provide anything to compensate for bad RNG. If you take one of the other talents things feels bland because you are doing the same thing over and over until Cd's come back up. Divine Purpose also make Inquisition feel better because of its ability to proc a free one.
Another option would be to make the Ret t16 4 set bonus baseline. Which is something I think would be pretty cool as someone who misses Divine Storm being in the single target rotation.