I would more say: Hero-Classes that may only fit for one race lorewise and where opening it for others would obliterate their whole style. Blizz did it before with the Tauren and it was shit.
That's why I would say, instead giving Wardens to the Horde, it would be better to give them a Class that fits the Lore of one of their races and only has the same mechanics. Like Wardens on the one side, Spellbreakers/Blademasters on the other.