My first thought was either Cole or Victor as well, but I honestly don't think they can be on GH since those characters are owned by Prospect Park, and especially since PP and ABC are in a big dispute regarding Todd/Starr/John right now. So, I though Heather, but then they showed her at Olivia's door. So, I thought Faison, but I don't think Anders Hove is coming back. So, IDK, maybe Helena? I honestly don't know.
I was thinking it could be Cole if prospect park wanted to use the character because they own Cole but if he's dead they can't use the character so it would sort of work that they ask GH to revive him. If not then I have no idea who it is.