One of the main themes of One Piece has always been “inherited will”, and for those of you who have seen those popular theories, will understand how strong that argument is. The greatest symbol of this concept is the famous “Straw hat”, which was passed down from Gol D. Roger, to Shanks, and finally to Luffy.
- The story of One Piece focuses heavily on the older generation making way for the generations of the future. This can be seen with the Supernovas making names for themselves, trying to take out the Yonko and very possibly taking their positions. And also with the Navy, where the old generation of Garp and Sengoku have stepped aside letting the next generation of admirals proceed onwards. Due to this focus, it is very likely Shanks will not be a Yonko while Luffy is Pirate King.
- Shanks is TOO powerful to be an ally to Luffy in the grand scheme of things. If Luffy faces adversity like at Marineford, there is always that chance that Shanks can show up and save the day. That does not seem like something Oda would want for his protagonist who is trying to be the King of the Pirates.
- Luffy is currently not strong enough to be the Pirate King and needs another push to get more powerful. The initial push was not being strong enough to save his brother Ace, which propelled Luffy to who he is now. However, his current strength is not enough to take on Yonko. With the death of Shanks to a rival like Blackbeard, Luffy gains even more hatred of Blackbeard and the kick he needs to get even more powerful. Shanks has already shown how dangerous he believes Blackbeard to be, and the mark on his face is proof of that.
Based on these three reasons, it is highly likely that Shanks will be dead before Luffy gets a chance to give the hat back. There are people that believe that Luffy would return the hat at a funeral for Shanks, but I believe something else. This come back to the themes of inherited will and moving from generation to generation. Luffy was motivated to be who he is by being around Shanks; Shanks eventually became a Yonko after originally being a rookie pirate under the former Pirate King; and something has to continue this going forward for the next generation. I believe that once the secret of One Piece is revealed, Luffy became Pirate King and the Straw Hat Pirates journey is over; the Straw Hats will separate and return to their individual homes. Waiting for Luffy back in Foosha Village is Makino and her child. This child will look up to Luffy, and Luffy will take interest in him too.
*Theory by UrsaKing