(SPOILER)Regis dying was known before even Kingsglaive was released. I'm pretty sure they even mention the fact during the Uncovered Event. I mean we also have been shown gameplay footage where Noctis and crew finds out about his death.

As for in the main game, who will die, I have a strong feeling that it will be (SPOILER)Lunafrey from how she talked in Kingsglaive about her destiny is to make sure Noctis fulfils his. As well as the fact that she's an oracle and while they are departing from the FFXIII it is a remanent from th Bersus days and well, we know how that goes. Or the twist is Noctis' destiny is to (SPOILER)die to save Eos or he dies in Luna's stead .

I dunno if my later tags were necessary...