First, Foreshadowing is about how certain details that seem irrelevant at first are actually important to future plots/arcs. Let me list some of the actual impressive foreshadowing in the series.
• Haki
This powerful technique was named and explained late in the series, somewhere in Marineford but is actually been foreshadowed in the very first chapter (Shanks using CoC to scare that seaking).
And the very first foreshadowing of Observation Haki was through Zoro way back in Alabasta when he’s in a duel with Mr 1. Before we learned about this technique, I thought Zoro was meditating or something (no joke). I think the Armament Haki one is when Garp hit Luffy with just his fingers but Luffy screamed in pain even though he’s immune to physical attacks.
• Luffy’s Ability to Hear All Things
It was revealed in Fishman Island Arc that Luffy could hear seakings and later in Zou, could hear Zunisha. He has the same ability with Momonosuke, Roger and Oden to hear all things.
But it was always evident in earlier chapters that Luffy could understand animals (First animal encounter is Shushu, the dog who defended his deceased owner store during the Buggy pirates saga). I didn’t question it cos maybe Luffy’s instinct and empathy was just unbelievably strong. But he literally understands them all along. When Chopper joined the crew, he became the animal translator so no one suspects anything about Luffy understanding animals. However, there are still subtle scenes that showed Luffy understands animals even though Chopper is present (eg. That camel in Alabasta and Karu).
• Going Merry’s Death
Merry’s first major damage was during the Reverse Mountain arc where his head was cut off due to impact and his mast broken by Luffy. Since then, Merry had been through various damages but was always repaired by Usopp. His damages were just taken lightly cos he’s just a “ship” and can be repaired. Not until in Skypeia when Usopp saw someone fixing him. But everyone was curious about who that person was not about Merry’s worse condition. And yeah, the inevitable happened, Merry’s death at Water 7. Literally no one expected to cry over a ship.
• Roger’s message at Skypiea
The foreshadowing is not about the message but about his knowledge in writing an ancient text. Nico Robin questioned about it for a good few seconds but then proceed to wonder what the message is all about. So the fact that Roger could write a poneglyph text was forgotten especially when Robin’s past was revealed. Robin as being the only one who can decipher poneglyphs buried that fact. But it was revealed in Zou that Roger had Oden as one of his crew members, a Kozuki that could write and make poneglyphs.
So way back in Skypeia, it was foreshadowed that Roger also has someone who can read and write poneglyphs. On top of that, he had a freaking Kozuki in his crew.
