I'm pretty convinced that Rey will turn to Dark Side in VIII and Kylo Light Side. There are too many factors could trigger Rey's transition, such as learning that Luke is the father who left him on that planet or possible death of Finn (remember, attachment is banned in Jedi philosophy). She had been full of fear and anger just like Anakin when he was a little child.
On the other hand, Kylo always had -and still has a family and friends, there are still strong ties in him to Light Side. He's just being manipulated by Snoke and a possible death of Snoke (I expect this to be done by Rey) could severe all his ties to Dark side.
Best possible scenario for me:
- No first order, no Republic, whole Galaxy is in chaos
- Snoke, Rey and other Knights of Ren are on the loose
- Luke is unsure about Rey's training yet she is a powerful jedi even without a proper training
- Both are searching the whole galaxy for Ren and Snoke to end the chaos
- Luke thinks Ren can be saved. They meet, Luke defeats Ren and captures him. There is another Knight (just like Dooku in Ep 2) and Rey fights with him and gets defeated.
- Remnants of Republic fight a team of powerful gangsters and scroundels who are under Snoke's manipulation (they may deploy battle droids just like Trade Federation) and defeats them
- Ren unites with his mother and cries for his father. He helps them to find Snoke.
- Rey, Luke, Finn and Ren fight Knights of Ren and defeat them (they surrender) Snoke captures Finn, reveals the truth about Luke being Rey's father and kills Finn. Bursting with anger she kills Snoke. An apologetic Luke tries to calm down Rey but it's a lost cause, her transition to dark side complete.
- Rey escapes with surrendered Knights of Ren.
- Finn's Funeral. Ben and Leia together. Luke is devastated. The End.
I haven't thought much about IX but it'll be probably about Luke and Kylo trying to find a way to stop a furious Rey who is trying to reinstate a republic on her own with a little help from Knights of Ren. The beautiful part of the IX will be the difficulty to tell who is actually dark and who is light side.