Personally, to me, the easiest way to fully restore You-Know-Who would be to let some of 7's nanoprobes tinker with B-4's positronics for a while. I imagine that would repair (or would it be upgrade?) any abnormalities and shortcomings of his more... primitive matrix, allowing Data, or at least the memories, and presumably some aspect of his identity, to surface more permanently. I mean, look at what happened when some of those nanoprobes got hold of the Mobile Emitter. A positronic brain isn't 29th century tech, for sure, but I imagine some creative stuff could come out of a fusion of Borg technology and Soong's creations...
Of course, with the fallout from Destiny, that might not be an option anymore...
That being said, I'm not necessarily comfortable with the idea of Data simply 'emerging' from B-4's mind... For one, that wasn't his goal when he downloaded his memories and experiences into B-4... it was to serve as a guide to possibly inspire him into 'extending' himself in the same manner as Data himself did. Those experiences were probably never meant to manifest as a full consciousness... That leads to unfortunate implications when you consider the events of "The Schizoid Man" where Ira Graves does essentially that very thing to Data himself, and we, as the audience, are meant to be outraged that he would do such a thing... Does Data have some right to hijack B-4's existence, no matter how limited, on some basis of being 'better'? Data was way too much of an ethicist to abide something like that, in my opinion, so I'm not really sure how one would go about restoring him...
Of course, with the fallout from Destiny, that might not be an option anymore...
That being said, I'm not necessarily comfortable with the idea of Data simply 'emerging' from B-4's mind... For one, that wasn't his goal when he downloaded his memories and experiences into B-4... it was to serve as a guide to possibly inspire him into 'extending' himself in the same manner as Data himself did. Those experiences were probably never meant to manifest as a full consciousness... That leads to unfortunate implications when you consider the events of "The Schizoid Man" where Ira Graves does essentially that very thing to Data himself, and we, as the audience, are meant to be outraged that he would do such a thing... Does Data have some right to hijack B-4's existence, no matter how limited, on some basis of being 'better'? Data was way too much of an ethicist to abide something like that, in my opinion, so I'm not really sure how one would go about restoring him...