I feel it may happen while fighting Kaido. Someone is going to, almost, die while fighting Kaido.
Law, in return for Luffy saving his life and defeating Doflamingo, will grant this person eternal life by using his “immortal operation”. And let’s face it, if it wasn’t going to be used, why would it have been mentioned in the first place??
Oda likes to drop little hints through out a few past events that’ll be important to some kind of huge future event; for example Luffy understanding Ryu form Warship Island Arc leading to him hearing Zou’s voice or Usopp telling his fake stories for them to only come true in Dressrosa and so on.
Oda has already split the team up before and is now splitting them up again, if in that case, maybe someone else will die for Luffy, again. Just this time he won’t go blank like he did when Ace died, which will probably be the harshest death there will ever be in Luffy’s life.
I say this because Law’s life goal has already been reached and he has nothing else to do. I understand that as a child he always lived as if he had nothing to live for and is dying, for me, it still seems that way even after consuming the Ope Ope No Mi. It just seems to me that Law ‘s life is fading. He just really wanted to avenge Donquixote Rosinante’s death, which was a success when Luffy defeated Doflamingo. Unless he has deeper plans, as in destroying the World Government which I highly doubt because he was ready to die fighting Doflamingo.
Tell me what do you think.
*Theory by mondaymorningsam