i don't know if this could be a foreshadowing, but in the chapter where Baskerville appear, there's a sentence:
'I've heard of three headed dogs before, but not men ' (or something like that)maybe it's just common knowledge of Cerberus, but who knows (Oda does of course)
Well, Baskerville was revealed to be three guys in a suit; This dog is a REAL three-headed animal, unless it's two smaller dogs perched on a larger dog's shoulders (Which would be stupid).
Some good foreshadowing is the Milky Dial.
Right here:
Usopp: Only the "Milky Dial" doesn't seem to work.
Box: Usopp Factory
Robin: It seems that being able to form clouds that way was a central focus for people up there…
I think this is foreshadowing of how they'll beat Smoker.