Priest has had meta-defining decks but not due to any synergy with their core evergreens e.g. Undertaker Priest, Shadowreaper Razakus & Dragon Priest. Blizzard always has to give Priest stronger cards in new expansions to retain its relevance. I think they should instead improve their basic and classic sets or give them a stronger class identity altogether.
The underlying problem with Priest is its lack of class identity and weak evergreens. This means it's always going to need strong cards from a new expansion because too many of their base core cards are too slow or situational.