I think the two remaining class will be "new" classes that recycle heavily from existing class.
My guesses are:
Borrows from Amazon, Demon Hunter, and Assassin
Uses (cross)bows for range, but must switch to daggers/fast combat/traps for melee.
Borrows from Monk, Crusader, Paladin
Whether using fists or weapons, the main emphasis is "holy". Probably also the shield using class?
Essentially, the existing classes are missing "bows" and "holy" which were huge in diablo2. I can't deny the appeal of a necromancer or how it would fit nicely into the lore. But I worry that a necro "pet class" would overlap too much with the druid pets.