Is there a chance? Yes.
Is the chance high? Probably not.
Adding in new classes/subclasses would be a significant amount of content (And with it would come a significant amount of bugs). Given that Larian doesn't want to make DLC or expansion content it drastically reduces the chance that such an undertaking would be done.
Still, Larian did add in some extra content to the Divinity games when they released Definitive Editions of them, so there's always the possibility that they make a Definitive Edition of BG3 which includes some additional content such as an extra class and/or some subclasses.
