Whatever it is that is happening, if there's one thing that is for sure, it is that Luffy (or Zoro for that matter), as a person will not dissapear from the story. The themes of this manga have been too much about Luffy wanting to be the most free person in the world and on having a grand adventure so that any form of predestination is basically out of the question. However, as the intro to One Piece has long told us, inhereted will IS a thing that can come into play and is it different from predestination in that one can carry the will of another without being that person or becoming that person.
Also I was thinking about the sudden logianess of Luffy's fruit and was reading on here it might be tree sap because rubber was made from tree sap so I was thinking if there were some special trees and then there was this:
In Chinese Mythology, a carving of a tree of life depicts a phoenix and a dragon; the dragon often represents immortality. A Taoist story tells of a tree that produces a peach of immortality every three thousand years, and anyone who eats the fruit receives immortality.
An archeological discovery in the 1990s was of a sacrificial pit at Sanxingdui in Sichuan, China. Dating from about 1200 BCE, it contained threebronze trees, one of them 4 meters high. At the base was a dragon, and fruit hanging from the lower branches. At the top is a bird-like (Phoenix) creature with claws. Also found in Sichuan, from the late Han dynasty (c 25 – 220 CE), is another tree of life. The ceramic base is guarded by a horned beast with wings. The leaves of the tree represent coins and people. At the apex is a bird with coins and the Sun.
If Luffy doesn't have the rubber fruit he might have the tree sap fruit from the Adam tree which is perhaps the "tree of Life", who knows what this might do when awakened. Having a healing effect doesn't sound too farfetched anyway. This myth is also very integratable with the Momotaro (peach boy) references and ties big players at the scene in both mythologies. The monkey (Luffy), the dog (Yamato) and the pheasant (that one is not so clear), the dragon (Kaidou and Momo) and the phoenix (Marco).
What is interesting here is also the interplay between the Gorosei saying that, even for them, (probably) Luffy's fruit is a legend because it hasn't awakaned in so long and the myth saying the peach only grows every 3000 years. In both instances, the special thing being that the fruit is very rare. The Peach of the momotaru legend gave birth to Momotaru or better, Momotaru was born out of the peach. Naturally Luffy will not be born because of his fruit but instead of being born, something latent within him might be awoken (the drums of liberation?) and this might be something that happens to anyone that can awaken this specific fruit, which happens very rarely.