While watching random youtube videos I came across the episode with Sanji on Kamabakka Kingdom. This Page from the manga is where my idea starts.
As you can see in this page from chapter 514 Sanji is talking to Ivankov about learning The 99 Vital Recipes. Ivankov goes on to say that they are a fundamental part of Newkama Kenpo, Art of Bridal Discipline. Or in other translations called Newkama Kenpo, Training for Married Life.
Basically Sanji learned FOOD RECIPES while training for MARRIED LIFE. Now it may just be me, but that sounds a lot like Oda foreshadowing the Whole Cake Island Arc.
I made this in a hurry, but I’m thinking maybe Oda Foreshadowed more New World Arcs in the different members training. Off the top of my head I’m thinking:
Zoro- Area he trained was a war torn country. Wano has been talked about as being ravaged by Kaido & Others.
Ussop- Everything was giant where he was sent. Elbaf Arc is supposed to be about giants.
Franky- Studied in a futuristic country that was Vegapunks original home. Could be talking about Punk Hazard or a futuristic arc with Vegapunk.
Nami- Studied advanced weather science on a sky island. Also may be a foreshadowing a Vegapunk Arc or another Arc on a sky island.
Robin- Worked with the Revolutionary Army. Reference to a Revolutionary Army Arc.
Chopper- Was on an island where giant birds and man fought. Honestly at a loss for this one no idea.
Brook- Became a famous musician. Also no idea.
*Theory by ZeroJeMoc