The last director, (Rian, I believe) left things in such a mess at the of TLJ, I'm not quite sure how JJ is supposed to pick up the peices.
I mean, unless she's CGI'd in, there's no satisfying way to explain Leia's presence, or lack thereof, in ROS.
The resistance is in shambles, and it would easily take 2 or 3 movies for them to even get to the point the Rebellion was against the Empire.
Not to mention others, but these 2 are the most glaring issues JJ has to solve.
There's no way he's going to be able to do it well in 1 movie. I could be wrong, and we'll all be pleasantly surprised, but I doubt it.