because Ryuuma and Zoro both use swords to fight, so Fujitora and Zoro would be a likely match in the future. Doctor Hogback was Chopper's idol, Sanji fought Absalom 99% of the time so he was technically his opponent. How I determined the uknown admiral would be up against Usopp is because they belong to the real make up of the SH pirates. The originals are Luffy, Zoro, Nami, Usopp and Sanji. The story also seems to center around these 5 indivisuals more often than the others. Chopper, Robin, Franky and Brook almost seem like side characters to fill up the core group.
Filler characters?!?!?!
Don't forget the importance they have for the history and how they have saved many dangerous situations. Also be sure that Robin at the end is gonna fight with a strongest opponent that Ussop's (or even Franky).