With so many characters becoming relevant this arc on top of all the quality developments I needed something visual to help myself understand this arc more intimately. It took me a while, but this is a summarised version of the players in this Dressrosa Arc at the moment.
Concerning the reason for why Oda-sensei excluded officially revealing Sabo, Sabo’s character may have some other surprise in store for the readers. With Oda-sensei laying on the developments thick these past few chapters, I wouldn’t be surprised if Sabo is actually the 7th Shichibukai, which would explain why Oda-sensei kept Sabo’s introduction for later. Sabo may not have been a well known pirate two years ago, but as shown by Blackbeard, it only takes one act to open the door to become a Shichibukai (and not to mention the World Government are totally bad at reading people – they failed to read Blackbeard and they are massively failing at reading the Luffy fanboy, Bartolomeo). With Kuma’s personality wiped out, the Revolutionary Army may have needed another member inside the Shichibukai.
Luffy doesn’t pay attention to the news much so he wouldn’t know about the new Shichibukai members. The other Strawhats don’t know the connection between Luffy and Sabo so even if they knew the identity of the 7th Shichibukai, they wouldn’t see the relevance in informing Luffy about it.
And just for good measure and laying on the developments thicker, I would love to see Oda-sensei bring in Jinbe to this arc.