I've haven't read this whole thread so someone might have mentioned this already: Joanna McCoy. I'd liked to have seen her. There's been so much fan speculation about her over the years, and if I remember correctly there was a script that never was. I would have liked to have seen McCoy as a father.
Often when family members or old friends turn up they've gone off the rails or there are recriminations and soul searching. I suspect this is what would have happened if Joanna had turned up. Part of me is glad she didn't since I would have liked to have see McCoy as a father without all that stuff happening.