Blackbeard, Akainu, and Shanks have barely actually gotten screentime throughout the series. Yes, they've had their "moments" but we know little to nothing about the background of BB and Shanks, who are arguably the most important characters in the manga at this point outside of Luffy and his crew. Im hasn't even been shown, and Akainu is clearly being setup as Luffy's adversary in the future.
Then you have characters who have barely been characterized like Dragon/Kuma or hidden characters like Vegapunk.
Just strikes me as odd to think BM and Kaido get all this exposure and not other, vastly more important or major characters. When Oda gets to them, I have a feeling he won't skip their backstories, its the one thing Oda prioritizes over anything else. Sure, he sometimes offpanels battles, but characterization he never seems to skip, even for the most insignificant.