Think about this logically guys. Oda decided to zoom on Izo of all people. If it was a Kaido feat, likely Kinemon would have likely gotten the reaction being the "leader" of the scabbards as we know it. If it was a Jack feat, Neko/Inu would have gotten the reaction. Queen and King are too busy dealing with the people below, so not them. It being Orochi/Kanjuro, etc doesn't merit Izo's reaction really.
Izo reacting is significant because he probably was chosen to react to something specifically. Yes, I acknowledge that it totally could be Kiku getting injured or something badly. However, they were all prepared to die coming up to fight Kaido.
The only things I could think of in this case are Kiku getting injured or a personal statement being made, that being "Ace" or "Whitebeard". Since both are dead, it couldn't be the actual character, BUT the card hierarchy in Kaido's crew allows for something like the Ace to be a thing, in name.
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my biggest guess and a better cliffhanger for the chapter is Kaido shout "Hey Ace!!" and Izo react like that
You and me both, it does make sense in the long run, but at this point I'm setting myself up for such massive disappointment lol.