If you had the power? Some don't want that, they want an exact replica of TOS and I kinda understand that. Here is what I would do, partially with Spock and Christine. Jess Bush tells us season two that Chapel and Spock develop a very close bond. Let's go with that. Let's say that that bond is very strong. Spock is visiting T'Pring on shore leave and Chapel is away perhaps with Korby and his team. There is an accident of some kind and she is severely injured. Spock is overwhelmed by how he senses her trauma through the bond. Even T"Pring feels it through Spock. She realizes what is going on and insists that Spock break this bond for good. Spock complies, and there is some kind of ritual somewhere but the effects on Christine are profound. Spock doesn't tell her. Chapel suffers a nervous breakdown over it and never truly recovers. This explains the wounded woman we see in TOS. It explains Spock's odd sense of shame whenever he's around her. It explains why he avoids her. She may never have any idea why this happened to her. Which characters would you retcon and how would you do it?