Structural lore — the ultimate ability of the Op-Op Fruit, why it is called the “Ultimate Devil Fruit.” Established Ch. 761.
What we know
Ch. 761 (p. 9): The Immortality Operation is the ultimate application of the Op-Op Fruit — the reason many call it the “Ultimate Devil Fruit.” A sufficiently skilled user can grant eternal life to another person, fulfilling “mankind’s longest and oldest dream.”
Ch. 761 (p. 9): The surgeon dies in performing it — the user loses their own life as the cost.
Ch. 761 (pp. 8–9): Doflamingo coveted the Op-Op Fruit specifically for this power and “had no intention of letting Law eat the fruit.” This establishes Doflamingo’s true motive: an immortality scheme built around the fruit. He laments that, had he held the fruit’s power “on that specific day years ago,” he could have used the treasure of Sacred Marijoa to “seize true world power.”
Foreshadowing seed (chekhov-explicit) — pending
Open questions about this topic
Nature of Devil Fruits — the upper bound of what a Paramecia power can do; the closest the series has come to devil-fruit-granted immortality.
Who Doflamingo intended as the beneficiary, and how/whether the operation is ultimately performed — forward.