Seriously stop sayin BS because you like a feature lol. No Gale as a Barbarian, Shadowheart as a Druid and Lae'Zel as a bard does not make any sense story-wise.
I also thought that multiclassing would have been enough... But it looks like Larian does not care to offer companions a narrative that doesn't make any sense for the sake of gameplay choices. Definitely something I'm not in line with.
That said in a comment : fextralife said that he talked about it with devs and that they were not sure if this would be in the game at release or not.
This is 100% on point. I really hope Larian lock the classes and subclasses of unique companions. If someone wants a different party composition they can get hirelings but the classes and subclasses of the story companions are untoucheable. At least we can't change their races...thank god for that small mercy because you know someone would take these companions and change them into horrific monstrosities in the character creation.