Okay, we all now this is not going to happen. This thread is more like a "what if... question". Nomura (I think) already said in an interview, that he wants to keep the Disney and FF characters out of the main narrative and only wants to focus on his original characters he has created. An even if this is not the case it was (I think) already official announced by Nomura, that all the new members will be "parts" of Xehanort like his Heartless and his Nobody etc.
But still, I think that Phantom Blot would actually fit quit well into the Organization XIII. He is already a hooded figure like them, wears black and is a criminal mastermind that pulls the strings in the back. Plus it would kinda be nice if at least one Disney character, some how, could actually tribute something to the story and be part of the main bad guys.
But like I said, this is just a "what if scenario".