I’m sticking to my theory that the term “regular cast member” is being used loosely and the list of the 8 core characters is pure conjecture on the part of the media.
However, if I were to entertain the idea that it is one of the 8 characters, it’s going to be Belle or Neal. They keep talking about Wicked versus Evil… so maybe part of WWW’s plan is to put Rumple in a position where he has to choose Belle’s life OR Neal’s life, but he can’t save both. Who would Rumple choose? His son or his true love? My guess is he saves his son, and Belle dies.