While reading the latest chapter 968, I realised that the voice was telling Roger that the king will be born in 10 years and will take another 15 to grow (not more than that otherwise the voice would have mentioned that).
Maybe this tells us how Poseidon will awaken her powers after 25 years but maybe just MAYBE it tells us how she will be used to do what she was meant for (hopefully destroying the World Government or something) in 25 YEARS.
Now, the 25 years have passed so maybe we don’t get anymore major flashbacks because it’s already time for Luffy to reach Laugh Tale and do what he’s meant to do.
*by Kromage911