IIRC speculation about Arthas appearing again in some capacity started back up with 8.0 when he had a new cinematic model in one of the cutscenes in Jaina's story. Then for SL, even before the Bluether animation released there were all the mentions of discovering who made Frostmourne and the Helm of Domination, callbacks to the design of ICC and Scourge architecture, etc. I definitely remember some pre-emptive groaning over some sort of possible Arthas redemption or otherwise dragging his character down, though who really knows what all was planned with how many times BfA and SL alike were rewritten or otherwise went awry.