Before we get started, please note that this theory contains major spoilers from One Piece Chapter 1044. Continue reading at your own risk.
I’ll keep this as short as possible. Since we know the fruit is the Hito Hito no Mi Model: Nika, I wonder if it gives this idea a little more credibility since we see the fruit is remarkable.
I’ll lay out all the manga panels and give a brief conclusion to make this easy to read.
Figure 1: After Roger finds the “One Piece”, he says how he’d love to have met Joyboy (ch. 967)
Figure 2: After returning from Laugh Tale, he tells Shanks something (ch. 968)
Figure 3: Roger tells Rayleigh that his son will be the one to reach Laugh Tale (ch. 968)
Figure 4: Who’s Who confirms Shanks stole the Gum-Gum Fruit (ch. 1018)
Figure 5: Lucky Roo also confirms they stole the fruit from an enemy ship (ch. 1)
Figure 6: Bald Gorosei guy saying they can never recover the fruit (ch 1044)
Figure 7: Shanks’ crew was docked on Foosha village for over a year (ch. 1)
Figure 8: Shanks makes it clear he has some goal or objective (ch. 1)
Figure 9: Roger entrusts his child with Garp (ch. 551)
Figure 10: Garp leaves Ace, subsequently Luffy, with Dadan on Dawn Island (ch 551)
Figure 11: Shanks entrusting his hat to Luffy (ch. 1)
Figure 12: Shanks saying Luffy said the same thing as Roger (ch. 506)
Figure 13: Momo tells us that Zunesha was a companion of Joyboy (ch. 1040)
Figure 14: Zunesha proclaims Joyboy has returned (ch. 1043)
So now I’ve laid out the relevant panels, what do they all mean?
Firstly, I obviously cannot speak to a character’s motivations, and we all know Goda can pull whatever he wants. Still, I’m simply making an observation that (obviously lol) I believe has some merit.
So let’s begin; Roger reaches Laugh Tale and learns the true history; here, I believe he also discovers the true nature of the Gum-Gum fruit; obviously, this being the Hito Hito no Mi Model: Nika. Upon returning to the ship, he divulges anonymous information to Shanks, it could be anything, but I’m not here to speculate on that. Roger then says that his son will be next to find Laugh Tale and the treasure left on it. Next, we know that Roger departs the ship to make a baby with Rogue and then hands himself over to the WG to pass on the torch to the next generation, not before entrusting Garp with the life of his son Ace, thus ending the life of the first Pirate King, Gol. D. Roger. Garp hands over primary care of Ace to Dadan on Dawn Island, East Blue.
Now, Shanks is off stealing the Gum-Gum fruit from Who’s Who; considering the unique nature of the fruit and the fact CP9 guarded the fruit on an escort mission, I can assume (I know, dirty word) that Shanks knew what the fruit was and how special it is. Shanks then docks in Foosha Village and conducts frequent voyages around the East Blue; this makes me think he went there with some goal or objective hence why he was there for a year. As we all know, Luffy ends up eating the fruit, and Shanks loses an arm, “Betting it on the new age”, as he put it to Whitebeard. Shanks entrusts Luffy with his hat and the goal of becoming a great pirate and sets off. I find it strange that as soon as the fruit is eaten, Shanks leaves because Luffy jeopardizes his original objective.
As we all now know, because of events in chapter 1044, Luffy has become Joyboy because of the awakening of the Hito Hito no Mi. Shanks stole the fruit and went to the East Blue to look for Ace to give him the fruit so Shanks could pass on Roger’s will to his son as Roger wanted. However, after Luffy ate the fruit, Shanks saved his life, and Luffy “said the same thing Roger did” (obviously, we don’t know if this was before or after eating the fruit); Shanks decided that Luffy could inherit Roger’s will and fulfill the Will of D.
TLDR: Shanks’ original objective was to steal the fruit and hand it to Ace. He tried to pass on Roger’s will to his son and carry on the Will of D. Instead, Luffy ate the fruit and became the one who would bear Roger’s and eventually Ace’s will.
