He might keep the Rocks flashback reserved for BlackBeard.
Maybe, but there are things about Kaido's last that are relevant to this arc specifically, which in still waiting for. For sure, the Rocks relevancy doesn't need to be addressed entirely, but I think there are things we still need answers to about Kaido himself (his lineage, his relationship with BM, getting his DF, potentially meeting Yamato's mother, his eventual infatuation with Wano). Obviously, nothing is guaranteed, but there are just things Kaido's says that make it odd Oda isn't diving deeper into them.
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I don't think a Kaido's flashback without Rocks would be satisfying, as this period seems to have shaped Kaido's character quite a lot. Basically it would feel incomplete. Or Rocks's character seems to be related to Blackbeard and it would very Oda to keep Rocks hidden til we're in Blackbeard's arc. Maybe Oda plans to draw long Rocks flashback featuring several characters, including Kaido (like Law flashback also featured Doflamingo backstory)
I have a theory where this might go (with Rocks/BB and even Kaido), but need to see next chapter first. It really depends on Kaido's fate after this punch.