Well, Old Guard is already a negative trait. And leaders with this trait are used by minor nations who really do not have a choice. Of course, even if more negative traits were to be introduced, they could not be so debilitating as to make that leader worse than a default or else they would be a waste of space. That said, I do not see Paradox changing the traits in the game at this point and modders have no control over what traits are available (only over who has which) so I really do not see this as anything that would happen anyway.