(SPOILER)Roxas is Ventus. He's Sora's nobody only insofar as he was created when Sora released his heart to free Kairi's, but Roxas has Ventus's heart. It's stated in the Xehanort Reports in Kingdom Hearts II that Roxas has a heart--which is why he can use the keyblade. The only reason everyone is so insistent that he's merely Roxas's nobody is because until Re:Coded, nobody knew that Ventus's heart was within Sora.

I'm not holding out for a third nobody. There's no reason for it. Roxas is already Sora's nobody containing Ventus's heart. Xehanort, on the other hand, is a wild card altogether. He'll be returning, that's certain, but in what form? His nobody and heartless have been defeated, and so the body and heart should reunite in Darkness and be reborn. As Master Xehanort? As Terranort? Both? Is Terra freed and reborn without Xehanort in his heart? What of the Unknown from Birth by Sleep who wears the Organization's cloak before the Organization exists and wields energy swords not dissimilar to those used by Xemnas? He's confirmed canon. There's more mystery yet without throwing another Sora nobody into the mix.