Portgas D. Ace was killed off after he was released .. because he found the answer to the question “Should I have been born?” .. Which is what he set out to sea for in the first place.
Thus his journey ended, and Oda made a remarkable ending to Ace by killing him off because, from a story standpoint, leaving Ace alive would serve no purpose, and also because it completes Ace as a character.
The only other character I think that fills this condition is Trafalgar D. Water Law.
Law has taken down Doflamingo and his legacy, and as far as we know he has no “journey out at sea” left for him. Thus his only remaining purpose (from a character standpoint) may be to revive Luffy if Luffy dies in Wano, or to give Luffy motivation to get stronger with his death, etc.
I’m not insisting on the method in which he dies, but I am certain that he will die … which would complete his character.
The greatest power of the Ope Ope no Mi is the ability to grant another person eternal youth via the “Perennial Youth Operation” though performing this feat comes at the cost of the current fruit user’s life. Introducing such a thing and not using it ever in the series would be foolish. And I’m sure Oda did it for a rason. Law will use it in the future and will die. So will he use it on Luffy?
It’s been quite a while since we’ve had a significant loss: Ace died almost 400 chapters ago. Trafalgar Law first appeared in chapter 498, and we are currently on chapter 961. He has been on a long ride with us the viewers, we got his sad back story and while not a member of the crew he has had about the same screen time as any Straw Hat and played a major role on the fight against Doflamingo. If a significantly emotional sacrifice has to be made, it has to be Law, as he is so important to us by now that his death would be devastating but it would still allow the Straw Hats to move on while keeping in mind of the people that died in their path (Whitebeard, Ace…)
I think Law will die through sacrifice to make a parallel with Ace’s death and to show the Ultimate Devil Fruit power. Luffy will literally die at the hands of an enemy (possibly Kaido on a 2nd round) and Law will use his power not to give him eternal youth, but to bring him back to life so he can fulfill Law’s final goal of bringing down Kaido on a 3rd round.
This will give us a once in a lifetime opportunity to see the Straw Hats’ devastating reactions to see their captain literally dead along with ours, since Luffy has been close to death many times but never died. I think it will also give Luffy a form of power-up as a “I can’t lose, Law sacrificed himself” since we saw how overwhelming Kaido’s power is compared to Luffy’s currently.
A whole event like this would make the Wano Arc intensity go above the clouds, and Oda himself has hyped this arc so much.
*Theory by BH_Shanks
