I think you're making a major mistake in trying to guess things like this based on niches in armor type or weapon type etc that are not yet filled. Blizzard goes with what seems cool and fun and exciting and as they've said specifically considering hero classes, with what seems appropriate for the lore of the expansion. Which is why they've done 2 melee classes in a row when everyone said the class after DKs could never be melee because melee had already been done.