Cloud, all the way. If Sora and Cloud fused together, then maybe Sora would get some kind of Buster Sword type of Keyblade or maybe he'd get the ability to perform that float-and-slash attack that Cloud does in KH. Besides, it'd be cool to see Sora with a black wing sprouting from his shoulder, heh heh.
Or, what if Sora eventually fused with Riku? Eh? Eh? Now THAT is something worth thinking about. I'd love to see a fusion of those two.