Johnson said “I wasn’t given any directive as to what that had to be...I was never given the information that she is this or she is that.” so I'm not sure, if it was a key plot point JJ had in mind like an 'I am your father!' moment I'm sure he would have told him.
We'll never know, maybe he did, and Rian just binned them, or maybe JJ had a vague idea whilst filming FA but it wasn't important enough for him to insist on it.
Pegg talked about Rey having a "relevant lineage" which, to me, is the worst thing they could have done...
His reasoning behind it seems pretty sound as well...
"Rey learning that she was related to someone like Luke Skywalker would have been “the easiest thing she could possibly hear.”
“The hardest thing to hear is, ‘Nope, this is not gonna define you,’” Johnson added. “And, in fact, Kylo is gonna use this to try and undercut your confidence so you’ll feel you have to lean on him for your identity . . . you’re gonna have to make the choice to find your own identity in this story.”
I think having a show runner with a clear vision for how the trilogy will play out would be a good thing...depending of course on the showrunner.