One Piece has always been good at weaving small details into larger fates, and Water 7 might include one of the more subtle foreshadowings of Usopp’s eventual progression in the story. Usopp began life as a cowardly storyteller who often told lies that became truth, and as he evolved from a storyteller to the “brave warrior of the sea,” his foreshadowing began in the early arcs of the story.
Water 7, in particular, did a deep dive into his identity with Sogeking, songs, and the theme of legends formed from imagination. With Oda putting so much emphasis on imagined myth, including in dreams and stories, a subtle detail from Water 7 might contain the groundwork for Usopp’s arguably biggest evolution to come.
Disclaimer: The article is a speculative theory and includes spoilers from the One Piece manga.
Explaining how Water 7 may foreshadow Usopp’s ultimate transformation in One Piece

Usopp's dream to become a "brave warrior of the sea" is one of the most consistent through lines since the start. Although Usopp is frequently the comic relief or cowardly character, Oda has made the point that imagination, myth, and stories can affect reality within his world.
In Water 7, Usopp adopts the alter ego of Sogeking, a completely fictional hero that exists only in his imagination. In spite of it being a fiction, others regard Sogeking as "real," and the spread of Sogeking's legend throughout the place affects even the children, the locals, and their notions of Sogeking.

This is a direct call back to Oda's own larger theme that "everything human beings can imagine is real", not to mention building upon this later when Vegapunk explains that Devil Fruits are born from a person's dreams and desires. In this sense, Usopp's skill set as a myth-making storyteller is significant because it gestures toward his power to make imagination turn into reality.
Another hidden layer within Water 7 is the way that songs and tales are passed on. Sogeking's theme song, sung openly in the city, perhaps continued to exist beyond what was revealed in the manga. The fact that songs like "Binks' Sake" pass down through generations indicates that Sogeking's legend also may have passed on via children and culture to become something greater than Usopp himself.

If Oda chooses to revive this, it would help in empowering Usopp at his low moment, when he draws confidence from the fact that the world already perceives him as the courageous warrior he aspires to be.
This may ultimately result in a mythical transformation, perhaps one linked to a Devil Fruit expression tied to his character, like the imagined "Human-Human Fruit, Model: Sogeking." Whether or not Oda goes that far, the symbolism is still strong: Usopp's falsehoods creating his reality, with Water 7 being the first major step of that notion.
His final transformation is perhaps not through brute force but through becoming the hero he once pretended to be, realizing his dream in the most Oda-like manner, through the strength of stories becoming reality.
Final thoughts

Usopp's future may not be strictly about strength, but more about Usopp embodying the myth he created in Water 7. By combining imagination and legacy together, Oda may show Usopp's growth into the "brave warrior of the sea" through Sogeking's legend.
Regardless of whether this involves a Devil Fruit or not, the message is clear: Usopp's greatest power comes from making belief into reality. Water 7 may have innocuously been the ground zero for his ultimate transition.
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