Xebec D Rocks is going to be a fishman. The first fishman with the will of D, alongside a giant thus far the only ones from other races. Xebec is probably inspired by the legend of the Roc in Sinbad, a legendary and enormous predatory bird, so I think he’ll be a flying fish. Using his fishman jujutsu he’ll surge through the updraft and use the crushing waves along with his terrifying momentum to destroy everything in it’s wake.
As a person I think he’ll be a blend of Luffy and someone more sinister like Hody Jones, being a guy who yearns for freedom and race equality - unlike Hody who is just blindly pursuing the intérets of his own race - but he shares the latter’s brutality and no-quarters-given attitude to where the goals justify the means to where he will have no qualms about killing those who dare stand in his way. He would ruthlessly eradicate the enemy forces trying to undermine his great utopia. He is dedicated like Hody and has perseverance like him to see it through.
Blackbeard ultimately doesn't fully get Xebec, kind of how Hody Jones admired Fisher Tiger but nevertheless took a course of direction that would be diametrically opposed to what Fisher Tiger stood for. Xebec is a guy who wants to create a utopia, whereas BB is driven by a will to dominate, and so sees Xebec as an inspiration insofar as him being able to do what no one had ever successfully done before him, exactly what BB is ultimately preparing for: challenging the very power of the WG itself. Whereas Xebec would have wanted to institute a democracy and world unity and all that in the aftermath of toppling the WG, Blackbeard ultimately wants to be the guy at the top.
Here in info about the Roc from Sinbad
https://en.wikipedia.org/wiki/Roc_(mythology)
