Well, nothing is more important to classpecting than cute little clues left almost definitely nonintentionally in the narrative. I think Witch and Sylph are active/passive counterparts because Kanaya says that a sylph is Sort Of Like A Witch But More Magical (of course, a witch channels and interacts with magic, but a Sylph, being an elemental creature, IS magic! Which suits a class known for its passivity.) And I think that Maid and Heir are counterparts because it would be cute if John and Jane had counterpart protagonist classes--also consider that a maid and an heir are conceptually linked, as servant and scion of magical nobility.