Currently game present us that option: 1 Thief 1 Cleric 1 Fighter 1 Mage 1 Warlock
We have lack of options in Baldur's Gate 1 or 2 there was more space in party and mroe characters to chose. In Divinity Original Sin and BG 3 we are limited to 4 character but in DO we got option to change every of character in our party from scrach almost everywhere. So if Larian want stay with 4 characters we shoud get a lot more avaible characters in later game. Baldur's Gate 2 for example allow player to join 5 from 17 avaible characters to his party. This was enought to make really ncie setups for every kind of player.
I believe it will be the case in the future. Swen talked about it in an interview, and they definitely intend to allow the change of classes for companions.