I feel like this would be good as well, if not at launch then at least as a patch down the road. Probably wouldn't be a good idea to change subclass though. While it would be great for customization/replayability, I feel that those who don't want this to be "Divinity Original Sin 3" would be disappointed.
I don't want DoS3 but I'd really like to be able to change my companion's subclass if it does not conflict with their story.