You guys do know that neither JJ nor Rian were simply able to make any film they want? They get ideas, scripts whatever and have a quite reasonable amount of freedom, yet it all passes through some committee with all kinds of questions and issues...
So, in the end, I don‘t think one was stuck because of another. They did have different styles and visions on how to keep going, but in the end, it is going to be a coherent, last trilogy. I‘m pretty sure they knew JJ would direct the last one, before shooting ep 8 even started, thus there certainly had been instructions made. (Why they didn‘t leave it to just one director will probably stay a mystery for me)
That‘s why I believe, that Episode 9 is the big revelation... tabula raza episode which changes the Star Wars universe completely...
And interestingly enough, apparently Lucas mentioned that Episode 8 was pretty close to what he would‘ve had in mind for Episode 7.
Theoretically, it‘ll be the end of the Skywalker storyline, handing the torch to a newer, fresher concept (favorably some grey force user style generation, where good and evil depend on the perception and side from which they are being looked at)
Yet, some villain or villains ARE needed. So, either it will be reduced to a "XYZ - A Star Wars Story" franchise or they completely start anew. Otherwise it‘ll repeat the evil vs good force user plot over and over again. Actually, a Yuuzhan Vong style saga/storyline might be refreshing.