I'm just going to re-read chapter 1137 and see if anything stands out to me.
1. It's possible that the ruins of Aurust Castle are a metaphor for the entire One Piece World. A huge, thrilling and terrifying place. Things are falling apart after a big battle. It's covered with skeletons, like we saw on Onigashima in Wano.
2. King Harald's portrait in this chapter COULD be a metaphor for something similar we might find on Laugh Tale for Nika/Joyboy. Another mural or other piece of artwork perhaps? Something that tells us more about the history of the World. Harald removed his horns, so maybe Nika/Joyboy did something similar. Possibly removing his "wings", if he does turn out to be a Skypiean or Lunarian.
3. Loki and Hajruden's relationship could end up paralleling Joyboy and Davy Jones' relationship. Maybe explaining the two "branches" of the D Clan that are carried onward by Luffy and Blackbeard.
4. All of Aurust Castle is being overgrown by parts of a tree. It's possible that the tree, itself could be the treasure?
5. "One day I'm gonna unite all the giant tribes!" This could parallel Joyboy or Luffy's dream. Only instead of JUST the Giants, it could be "Uniting ALL people".
6. Gunko's tall shoes COULD be a metaphor for the Red Line.
7. Gunko's "Trajectory of the future" could hint at the difference between Imu and Luffy's ideals. One believes the future is fixed. The other believes that the future can be changed. Gunko is able to wound Loki, but her attacks don't kill him. Just like how the followers of the D Clan have been persecuted, but not outright defeated.
8. Shamrock is shrouded in shadow before his reveal. This feels like a similar look that Imu has. So, Shamrock might be a parallel for Imu, in this chapter.
9. They reveal that the World Government are after the Giants, themselves. So, the "treasure" of Elbaph that the World Government wants to steal...is actually it's people. So, maybe the hint is that the One Piece Treasure actually IS all the people of the World? Or a very special race that's been hidden away?
10. Luffy is after a key to free Loki in Aurust Castle. So, while the Key IS the treasure Luffy is seeking, it's not the "main" treasure. What we find on Laugh Tale might help us "unlock" the "true treasure" of this world. The One Piece Treasure may just be a way to unlock the "true treasure".
So, while I'm probably reading too much into things, I think there's two strong possibilities for what the hint Oda's talking about are.
1. It could be the Tree itself. It's everywhere in the Chapter, but it's overlooked in the background. There could be some sort of important Tree on Laugh Tale, to complement all the other important trees we've seen throughout the story. Something that's everywhere in the background, but everyone is overlooking it. It's possible that Oda's doing something with nature. Something similar to the situation in Nausicaa of the Valley of the Wind. Where humanity changed nature in order to heal the damage done to it, but now need to change things back in order to complete the process. Humanity "playing god" too much.
Or
2. It's everyone in the world, or another secret race of people, in order to parallel the Giants. The World Govenment wants to rule the Giants. Luffy wants to free Loki so that he can get information he wants. The "Giants" are the treasure of Elbaph. So, maybe Luffy will need to free another race of people that the WG wants to rule over? Possibly, either more Lunarians, or to go along with my theories...the Dragon race. "People" are likely the "true treasure", one way or another. The "key" is Luffy's goal, but what you can unlock with that "key" is likely more important to the World at large. Just like how the One Piece Treasure is Luffy's goal, but what you DO with the Treasure is likely more important than just finding it.
I'm personally leaning towards #2. But, we'll see. It's not impossible for both things to be true. Especiallly if all these Giant Trees we see around the world are actually some sort of transformed race. Like, the Toys on Dressrossa. There could be a race of people who got transformed into trees in the distant past? I've thought similar things about the Transponder Snails before. That's not impossible in One Piece.
But, that's my best guesses. It's possible that I'm missing something obvious, here.