I think it is possible that the actors probably want one more season and then that is it and that could be a sticking point. I also think that they are unhappy with where the plot has gone. Both Jennifer and Robert have said that they were upset about the loss of Neal. I think you can see them trying to reel Jennifer back in each year, with the Frozen plot which was pretty Emma heavy, and then the Dark One plot which then ended up not being about Emma. I think she has been repeatedly disappointed and she wants it to be done or wants to know that there is an end in sight now. Like she said, Jennifer likes her character and wants to finish her out, but she is ready to do other things.
I think the fact that they are still talking makes it much more likely that we are going to lose Jennifer. She wants something and they might not be willing to give it. And if the three of them, Jennifer, Lana, and Robert have some sort of sticking point then it seems even more likely that Jennifer might be done. She has the least reason to stay at this point. She doesn’t have kids and she has other opportunities. I think Robert has been done for years, since Neal died, but as he has said in the past, it is a good pay check and his kids go to school there, so it is much more complicated for him.
For me though, it is much more complicated to talk about the plot reasons for who those two new characters could be. They could be anyone. They could be an adult Henry or adult baby Neal. It is also possible that they are just some new characters. They could be Hook’s son. They could be anyone because i really think it is going to depend on who stays and who goes.