Before returning to the present, it is possible that the flashback does not end six years ago, at the time of Loki's capture by Shanks, but continues until a much more recent period: the moment when Shanks came to Elbaf and visited Loki (just before defeating Eustass Kid).
We know nothing about the content of the discussion between Shanks and Loki, except for one thing: Shanks supposedly revealed to Loki his next destination (which logically means the place where he is currently located).
Luffy wants to free Loki to find out where Shanks is, to see Shanks again.
Since the beginning of the arc, this is actually the only real plot element that drives Luffy to act on Elbaf, to actively get involved in the arc's plot, and not just remain a passive observer of the conflict between the Giants and the Holy Knights.
If this element is also central to the construction of the arc, it is probably because the island where Shanks is located is the place where the next arc after Elbaf will take place. This island would then be the next destination for the Straw Hat crew.
Loki will likely be freed when he returns to the present, after he has finished telling his story, as promised, so he can help in the fight against the Holy Knights.
two scenarios are possible:
The most logical scenario would be that Loki waits until the end of the fights to reveal everything to Luffy about Shanks' location, at the very end of the arc. In that case, the identity of the island where Shanks is located would remain deliberately hidden until that moment.
The other possibility is the one theorized in this post, that the identity of this island is revealed directly at the end of the flashback, during the scene of the recent encounter between Shanks and Loki.
Which island do you think it is?
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