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?
People’s counter-argument is that Law stated that he wanted to find the One Piece. Maybe that’s true, but Ace also wanted to beat Whitebeard, and after joining him, wanted to make Whitebeard the Pirate King. Moments before Luffy was about to be killed.. he realizes that he is content with sacrificing himself to save Luffy and that what he wanted, what he really set out to sea for… he had already reached his destination. The same applies to Law.
*Theory by BH_Shanks