I honestly cannot tell if that final line is basically Oda saying "Settle in, we're gonna be in Elbaph for a LOOOONG time" or if he's saying "Don't expect the Strawhats to stick around too long guys, we got places to be". Because I can read it both ways.
The fact that it so nicely parallels the scene with the ship falling from the sky from Skypeia (c218) with Willie Gallon/Wily Karen is kind of making me think it's going to be a longer arc. (I kind of wonder if Louis Arnot is part of the same crew as Willie?)
But, we kind of blazed through the "Underworld" portion of Elbaph, already. And, for as nice as that double page spread is, the "Sun" portion actually looks to be the size of a small village, relatively speaking. If you count, it's like 12 houses and a castle. There's probably going to be something interesting in the "Heaven" portion. But, I could still see this being a relatively short arc.
Especially if there is some sort of time-dilation thing going on, with 1 day equaling a month outside the island or something, explaining the warning. The Strawhats probably wouldn't want to stay on the island too long, if that was the case. And there was SOME foreshadowing of something like that, since Luffy's team had their whole Lego adventure in the same amount of time it took Robin's team to just sail up to the island. Oda's not that great with time scaling, either though, so it could be nothing.
So, I dunno. We'll see, I guess. I'll admit, I might just be in denial of Elbaph being another long arc, because I do want to get to the big answers of this series, already.
As for who the silhouette is...you know what? I'm keeping my dark horse pick. I don't think it's ANYBODY we know. I think it's going to be a brand new character. I still think this could be Oda's version of Ratatoskr the World Tree Squirrel! That's not a drink! It's a nut!
https://en.wikipedia.org/wiki/Ratatoskr
(Yeah, yeah, "You're the nut!" Obvious joke is obvious. Now you have to be more creative.)
The line "Rainbows are meant to be crossed" feels like it's going to have some bigger importance later on, somehow.
Gerd was cute this chapter. She seems to be a bit of a human fangirl, too. I guess humans are just a little exotic on this island full of giants. At least for the younger generation.