Blizzard says a lot of things. The foreshadowing for Arthas is even thicker than the one for N'zoth back in early BFA. Nearly every single character connected to him is present in some form or another, and the Helm of Domination is a major plot device from the looks of it as well. Arthas will show up and I strongly doubt it'll be as a cameo of him saying hi to Kel'thuzad or something.