Is your assumption that Abrams just handed off a cliffhanger to Johnson and let him write from there? It's hard for me to believe that the three movies weren't plotted out in advance by Abrams, with Johnson being left to write the screenplay for VIII, much like the way Lucas plotted Empire but the screenplay is credited to Leigh Brackett and Lawrence Kasdan.
I spoke to my friend again, by the way. He said that he's seen a number of interviews with Rian Johnson and others, and they basically said that since JJ Abrams wasn't expecting to direct any of the other films in the trilogy he actually made no plans at all for later films - he introduced a number of elements which could be explored later and which were written in such a way as that future directors could take them in a handful of different directions, but agreed nothing with regards to what actually should happen. This meant that Rian Johnson basically had carte blanche, within reason, to propose anything, including things which fitted into his aforementioned vision and style in films, such as that Snoke's identity should not be the focus of the series but he should be "just another feature of the galaxy", and that Rey's parents should be nobodies.