Unused footage from Star Wars the Force Awakens will be used for the Carrie Fisher / Princess Leia scenes. It is difficult to see how unused footage could be used to significantly advance the movie plot. It would likely to be part of an onscreen farewell to Carrie Fisher who died in real life.
It is most likely that Luke will be Force Ghost.
It was established that Force Ghosts can trigger powerful lightning.
How will the Star Wars franchise recover from the Last Jedi?
The Han Solo movie was mostly irrelevant and suffered a boycott.
JJ Abrams is back to direct the movie will be out in December 2019 instead of May.
Among many issues is how to make Kylo or whoever else is introduced into an interesting and compelling villain?
How to re-establish the original Star Wars, Empire Strikes and the prequels as part of a journey which has a satisfying culmination in Star Wars IX>