I'm thinking of building a Priest deck and am missing Vol'gin, Prophet Velen, Cabal Shadow Priest, Shadowbomber, Mindgames and Shadowform. I have 2x all the other cards. Are any of my missing cards necessary? I know Cabal Shadow Priest is but the only other one I can see as useful is Vol'gin but he's not a card that would win the game? Any Neutral epic/legendary (other than the obvious like Rag/Dr.Boom) cards good choices?
Really only Vol'gin is needed (not even "needed", but very useful, since it combo's so well with Holy Smite, Holy Nova, Auchenai Soulpriest (either with coin or already on the table)). You not only clear their powerful card, but make one of yours even more powerful at the same time.
So many useful combos with priest, resurrect with Injured Blademaster, Light of Naaru with Injured Blademaster, double healths+inner fire on any high health minion = instant death many times, plenty of instant death cards, card copy abilities, Mind Control, etc... If you have a couple other legendaries like Sylvannas, Alexstrasza, Dr Boom, you are pretty much unstoppable unless you get bad card draw.
For other legendaries that are extremely useful, Emperor Thaurissan, Kel'Thusad, Maexxna, Fugen/Stalagg all come to mind, but you need to be careful not to stack your deck too high in the mana cost.