It's a good idea and much better design but defeats the purpose of a highlander archetype as that is effect is not worth having only one copy of everything in your deck. It's the reason why Inkmaster Solia or Krul never really took off in MSG meta as a mediocre one time effect is barely worthwhile but continual value in Raza is.
Blizzard would have to remove the "if your deck contains no duplicates" part of the card in order for it to be viable at all, which would mean highlander priest could run duplicates which would be really scary (double mind blast shenanigans etc). Obviousy they would have to cut Kazakus which is one of the most powerful aspects of the deck but i think duplicates or some other package would make up for this
I think a better solution would be to simply change Shadowreaper Anduin to cant target face to remove OTK bullshit which is crippling other control decks like pally and warlock atm and would take priest back to its control orientated roots