It will be Momonosuke. The only other plausible candidates were Luffy and Zoro, but they are presently caught against the Beast Pirates. There is that silhouette that was behind Jinbe and Robin, but I think that might've been a stowaway. Plus Jinbe and Robin should likely have other things on their plate. Namely the Road Poneglyph Kaido possesses.
The reason I believe it will be Momo is because he's the only major character on Onigashima with no immediate plan or purpose. It could be to try and regroup with the others, but he'll only get lost in the shuffle, rendering his capture pointless. Oda specifically introduced a son to Kaido for a reason, I don't think it's to be just a plot device to get everyone to meet their matchups. That could've been done without Yamato.
He will serve as a foil/companion for Momo.