Lore enthusiasts will be surprised most by Anduin's story in Legion. We will see a big leap in his development, with a young prince having to step up and make decisions that he hasn't been forced to make before. He has been looked at as a very pacifist character over the years, looking for peace between the Horde and Alliance, but as he gets older it won't be so easy. Players will come to understand his father's more aggressive approach. He is a character the team really wants to build out.
We will most likely be waiting to see more of Illidan until a later content patch. There will be a lot of Illidan at a certain point.
The Legion is tethered to something bigger and darker than the Old Gods.
Sargeras sees infestation and corruption throughout the universe. He has fallen to the wrong approach, but there are issues in the universe and his approach is to eradicate it all.
The current cycle of the game is in a place where we are facing really big and dark threats and only the heroes of Azeroth can save us.
The nature of serialized storytelling is that there has to be a big threat, but the team could take a step back and look at smaller stories.
Med'an is out adventuring in the universe, doing big important things that will one day com to fruition. He just isn't around right now.
There will be lots of Khadgar in Legion.
Waugh had no idea if Broxigar or Freya will appear in Legion.
Some heroes are dying off in Legion, but there will be a lot of changes with existing characters, with some being put in positions you never would have assumed they would be put in.