All they need to do is the following three things.
1. World scaling everywhere, not just legion zones.
2. World Quests everywhere, things like towns being under attack, rare mobs etc.
3. Re-tune mob dmg/hp. There needs to be some level of threat and mobs need to last for at least 10 seconds or so. Also make heirlooms give XP bonuses, not turn you into a demigod, and remove the ability to enchant them.
I've been lvling a new character, without heirlooms.... But since I know the game, it's still stupidly easy bar a few select quests where I had to get abit more creative.
There needs to be abit more danger than there is.