I did read your original post. It's Warcraft, no one stays dead.
Yeah I agree they bring in side characters. But if we're getting Two Princes in 5.2 (since it's Chapter III), we're going to need a hell of a lot of foreshadowing to justify Saballion over Anduin. One's been significantly built up over the past year or so, the other an Outland quest giver that barely anyone will remember.
Also, I hope it's not Saballion because more black dragonflight infighting is dull. I like the fact Wrathion's the last Black Dragon on Azeroth and isn't batshit insane. We don't need to introduce the "evil twin" trope nor more dragon politics. Cataclysm dealt with that fine.