Momonosuke should join the Straw Hat Pirates crew as an apprentice. There are three people in particular who he should learn from.
Zoro – He can teach Momonosuke swordsmanship skills. Which is very important since Momonosuke will need to become the leader of Wano.
Nico Robin – The Kozuki clan were once able to read the Poneglyphs. However they can no longer. But If Robin tutors him that tradition doesn’t need to end.
Chopper – Right now Momonosuke has no control over his Dragon abilities. If Chopper helps, he could use Rumble Balls to improve his abilities.
Another factor no less important is the relationship between Luffy and Momonosuke and how it evolved.
Luffy gives the boy a reality check.
Luffy treats the boy like a man.
First big character progression for Momonosuke. Kawamatsu takes notice.
Luffy’s proud of his little brother’s courage and resolve.
One of the most underrated part of the Saga. While Luffy’s been growing as a man and leader, he’s also been teaching Momonosuke how to grow up too.
Last thing I want to add is comparing Momonosuke to Vivi, where Vivi is an official Nakama but she’s not on the ship.
We could have a situation where if Momonosuke proves himself he can be an honorary Nakama.
*Theory by DiamondCoal / Lelouche85