I think Kizaru is saved for Blackbeard in all fairness. Blackbeard needs to prove himself above the admiral on his crew.
Imo I don't think Blackbeard needs to take out other characters and villains. He gained notoriety by his actions on a grander scale. He became a Yonko purely by stealing WBs power and knowing his territories.
He doesn't need to prove he's above anyone except Luffy when the time comes. What will probably make him more dangerous than Kaido will be his 3rd DF in combination with the others, along with what I believe is Pluton that he's going after.
No need to have him kill off Shanks or Admirals who deserve bigger screentime than that
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I like this theory, though I guess Oda would have to find a way to make Fujitora a bad guy again.
I don't think "Bad" is really the right term for certain fights tbh. He's just going to do his military duty and serve as an obstruction. In 956, he was still very clear with Akainu about wanting to take down Pirates, since he believes the SSG are perfect for it.
Fujitora likes Luffy for sure, but it doesn't mean he won't stand in their way when the time comes.