Someone new, and has a weird accent, and has a connection with one of the SHs. And has a "huge wound" in his back...
Let's use logicks!
-He's waiting for "that man", so that rules out Nami and Robin.
-He has a "huge wound", so that might rule out Sanji, Chopper, and Luffy, who don't really cause flesh wounds/scarring.
-So we have Usopp, Brooke, Zoro, and Franky left.
-Now, Duval said he's been waiting his "whole life" for revenge. So either he's a lot younger than he looks, or a lot older (having antagonized Brooke).
I've been thinking that myself but whenever I say something about it I get made fun of by others. He's the last person someone would think of, other than chopper, as a person to get back at for revenge.
Although, if Duval is going for revenge against chopper it might have to do with his first transformation into the monster point after eating too many rumble balls. Duval did say something about his scar on his back hurting.