As we know, Shiryu of the Rain Shiryu has eaten the Suke Suke no Mi, a Paramecia-class Devil Fruit which gives him the ability to turn himself and whatever he is holding invisible. He acquired this fruit soon after his crew murdered its previous user, Absalom.
With this fruit, he is able to cut opponents without being spotted, taking them by surprise.
I have this weird theory: maybe Shiryu will be Sanji’s counterpart on Blackbeard’s crew. Shiryu got the invisible devil fruit from Absalom. Way back on Thriller Bark, Sanji says that Absalom got the devil fruit he always wanted.
Recently we saw Sanji’s raid suit. Its unique ability is that it allows Sanji to blend in perfectly with his environment, leaving him invisible to the human eye. This invisibility gives him a great advantage in battle, being able to land surprise attacks on opponents without them seeing him coming.
If Sanji would master Observation Haki, then it would be good against Shiryu’s invisible devil fruit.
And also, since Sanji joined the Straw Hats crew, he and Zoro aren’t always getting along with each other. They always fight so maybe, this could be a foreshadowing in the future that Sanji will fight a swordsman toe to toe.
*Theory by Renz