I’m fairly certain aspects of the story were developed with the story group (and quite likely the TFA writing team), but there’s nothing wrong with Johnson going solo on writing duties. In fact, I think it will be interesting to see a singular vision in that regard.

I guess it’s possible, it would be interesting even if he just got to flesh out an existing plan for the narrative. But I’m just suspicious that this is all a Lucasfilm board room creation after the 2-3 named writers on TFA. It’s probably all just done with a committee.
“I am not a committee” - Rian Johnson, probably.