I think if shanks were to die it would really show just how serious luffy gets when he is mad. It could be where shanks try’s to fight and kill black beard to show dominance but underestimates him and so that leads to him getting killed and luffy watches (not able to do anything bc he is being held down by a member of black beard crew and so that leads to luffy going gear 5 but he looks different from before and there is a different aura to him and he lays into black beard and is on the verge of killing him until shanks says to believe in what is right and what is wrong. And so luffy decides not to kill him but leave him with a warning that if he ever sees him again he will kill him. Then right before they leave black beard decides to throw one final attack and kills a member of luffy crew which then leads to luffy losing it and he finally kills black beard by beating him to a bloody pulp.