The mystery box is all fine and dandy if you have a half-decent idea to back it up.
Except that nowadays most movies and shows are created during a brainstorming session.
"What if Rey has a mysterious past ?"
They throw out all these ideas, somebody shouts "This movie writes itself !" they pat each other on the back, have pizza and the next morning they task the intern to stitch it all together by 9am Thursday.
JJ has this thing for the mystery box and setting up weird and mysterious stuff has been a staple of a lot of recent fiction like Lost, Battlestar Galactica and the new Star Wars trilogy.
And in most of these cases they figure they can get away with somebody shaking the bushes making a spooky sound and dropping the odd weird hint.
The real problem is that the resolution is usually a "we'll figure something when we finally get there" type solution. And they either discover that the internet has already figured out every single detail and you have to pretend your 
cover story will confuse people long enough for them not to notice all you had was a dumb ending hotglued onto the final episode.
Now there was no way that Abrams would come up with a satisfactory answer to who Rey was, if she was a Kenobi or a secret Skywalker, it would be lazy, if it was something a bit more creative like a reincarnated Annakin, it would only raise more questions. And if anything it would make the saga into a kind of incestuous fanservice show.
Star Wars doesn't have much more to offer than lightsabers, space wizards and a constant battle between good and evil with a few droids and spaceships thrown in. And even if you tried something different, the fans would go crazy anyway.
Ryan pretty much did the sensible thing and nuked the idea that she could only be the secret daughter of a core character and that Snoke was going to have this really amazing mind-bending backstory and turn out to be the most unexpected character return like Luuke, Luke's evil clone, or it would be a really dumb one like "Snoke is really the second Ugnaught on the right and we'll have somebody write a novel to explain it all" Which brings us to the looming shadow of the EU, which has become a kind of impossible ignore Hadith to a bunch of vocal fans who will settle for nothing less than a stream of EU adaptations, because "Da buhks !!!"
The problem is that Abrams' mystery box setup caused more problems than anything. It was a well-made but badly written film.
Ryan at least had the nerve to cut the wires and come up with something the internet hive-mind capable of spouting a million possible story iterations every single second of the two year gap between films had foreseen, but did not retain. Ryan had much better ideas, but his better-written film was riddled with fragmentary story and exposition and shoddily made movie in general.
There is no way Abrams could come up with something that wouldn't disappoint on some level.
And the fan backlash is going to cause Abrams to come up with a dumb retcon "Yes, Rey is actually …" in an attempt to salvage the whole thing.
There is a reason Lucas outside of a few statements within a very narrow period never mentioned the sequels again, because he understood that no matter what he tried he would simply end up doing more of the same and while Lucas has his faults he at least tried to cover new ground with the prequels, his trilogy did have a good and definite end.
That's the big difference between Marvel and Star Wars, the former has a near unlimited pool of characters and possibilities, the other is always going to be about laserswords, eternal struggle between light and dark with some gimmicks thrown in.
paints a couple of mean-looking mutant-type villains from the far future.
