I wouldnt necessarily say that the two meeting are a given, though I do think it would be quite neat. Sadly story beats in WoW rarely happens just because they are neat, otherwise we would ahve actually gotten a Illidan/Malfurion reunion in Legion.
Reagarding Anduin and Arthas being MIA, I place about a 50% chance on either becoming a surprise brainwashed boss in a later raid. Anduin especially would be pretty amazing as a boss. Arthas would be nice too, though only if he isnt just a Death Knight again.