I will answer the second part first.
@AdmiralYonkouMt.Bandit:
and how is it truly brilliant?
We have completely random scene - unconnected to the events happening around- in which Nami is trying on some clothes. What was the relevance of this? Think about it for a second without considering the events happening after they left East Blue.
The first thing that came to my mind when I saw those was "hey, she looks really hot in those, why doesn't she wear them more often?". Then I thought, "is this how she is going to develop in the future?". Then I thought "maybe Oda is teasing us with rejected character designs".
It could easily have been any of the above, but later we see Robin in exactly one of the outfits. Anyone remembering that outfit would've definitely thought "Whoever she is, she is going to be damn important. So thats what that clothes scene meant". And we know that she went ahead to join the SH's.
A casual reader wouldn't even notice that random event. An avid reader could've easily made the connection with Robin when she was introduced. And only a truly crazy fan could've made a connection as soon as Hancock was introduced. This is where the brilliance is - a seemingly completely random scene like that having relevance a few chapters after the scene occurred and also having relevance years after the scene has occurred.
@AdmiralYonkouMt.Bandit:
As for the last part are you changing your stance to say that it's not foreshadowing that she will join but foreshadowing that some character would be wearing those clothes later on? Cuz the part where it's a clue that Hancock will join is what I'm talking about. If she doesn't join what exactly did that foreshadow?
The reason I mentioned it might be misleading is because, one part of the foreshadowing already happened (Robin joining), so the second part may be a red herring.
Now we know it did foreshadow Hancock joining for sure, but not at what level. Based on recent chapters, we can be almost 100% sure that she will atleast become an honorary nakama, like Vivi. Whether she will join the crew full time is what remains to be seen, and IMHO, still likely.