I'm going to guess the next kidnapping victim will be a cook. Next thing on the list is making sure you can eat, I would think.
Because, right now, it kind of feels like this is a dark mirror version of Luffy building up his crew? They were after a woman, like Nami. Then they were after a carpenter, which kind of parallels Usopp or Franky. So, next should be either a cook like Sanji, or a blacksmith/swordsman like Zoro. And probably a musician like Brook at some point. If this is at all going like I think it could be. It's hard to figure out a pattern with only two examples.
Personally, I kind of wonder if someone's trying to build their own version of Onigashima? Like, someone's trying to build their own villain fortress that Yamato gets a excuse to knock down? Maybe they're building a ship to get out of there. But, they kidnapped a carpenter, not a shipwright. Those are similar things, but not the same. So, I think they're building a building on land, most likely.
So, all that being said, I'm going to guess that our mystery kidnapper is...Scratchman Apoo. As far as we know, he's still on Wano. And he really didn't get a chance to do much during the Wano arc. I think this might be a chance to do a little more with one of the Worst Generation. He's building some sort of night club rave hangout and Yamato's going to have to literally bring down the house on him.