Yeah but a character is about a story and a background. What you suggest is basically BG3 having several Commander Shephard's that you can tweak into your liking instead of actual characters you can play as. If you want to play as a custom character thats fine, but Larian is already doing that. Astarion is Astarion, he is a rogue and thats it. Gale is inseparable from his Wizardry, he wouldn't even work as a sorcerer, because he is too much of a nerd.