There are 2 classes i can see them adding:
Monk-uses holy magic to base their moves around.
Demon Hunter- uses demonic magic compared to the necromancers undead. He could be a heavy armor user(i like to picture war from darksiders) and i could see him using a new weapon like a crossbow.
These two seem like they could separate themselves from the other classes well.
I know they sound like WoWs hero classes but don’t think of them like that.