I love the idea of bringing back Serena and Christina. I know Kin Shriner said on Twitter that TPTB doesn't have any interest in bringing back Serena, but it's so much this show is missing out on by not having her and her sister on canvas.
I would have Serena either be an attorney or a financial coordinator at General Hospital, and I would have Christina, or Tina, for short, be a doctor, falling in the footsteps of her biological mother Julie and former stepfather Kevin.
Also, Tommy Hardy is a no-brainer, making him a pediatrician like his mother, Simone. He even has a big name like Victoria Rowell or Vanessa Estelle Williams to play Simone on a recurring basis, while Matthew Ashford can be Tom Hardy again when needed.