This makes sense, I've seen people complain that they can't be a shark druid or whatever. There's companion subclasses for priest, monk, cipher, druid, chanter, paladin... I think that's all of them?
I think at least for Fighter, Rogue, Wizard and Barbarian we will get something new.
For the others, I'd love to see something new too, but I imagine they will use the companion's subclasses.