Force Ghosts of OB1, Yoda and Anakin...
Rey will ask Luke to train her. He will say 'no' because of what happened to Kylo. Luke will go off to a corner of the island and consult the Force ghosts of Obi-Wan, Yoda and Anakin. They will tell him to stop feeling guilty, get off his rear end and get in the fight.
They will have to use a CGI Alec Guinness of course, as well as a CGI Sebastian Shaw. If they even dare bring back Hayden, I'm out. Yoda will be a puppet, as evidenced by the story of them building the Ach-to sets off the ground to allow for puppeteers below (like they built the Dagobah sets back in the day)