I think this is the biggest issue is that Barbarian has nothing that really makes it unique and what was perfoming well has almost always been due to bugs, broken mechanics, poor design or Seasonal mechanics (Barber), or exploits like Snapshotting.
The Class Mechanic is incredibly underwhelming and lame. Shouts should not be Skills but rather part of the over-arching Class Mechanics and the Arsenal System needs to be overhauled. It was crazy that the 10% inherent Class DR was removed from Barbarian and then HP scaling was nerfed with removing the Temper. Fortify is lackluster and if it wasn’t for Weapon Mastery, is almost impossible to keep up.
What exactly is special about the Barbarian? Blizzard needs to focus on that. The Key Passives were arguably THE STOP GAP measure that helped make almost every build not horrible, then because of select builds being busted, Blizzard just arbitrarily raked them all over the coals.
IMO - Remove Shouts as Skills. Put them as Class Mechanic. These should always be active like Auras but changed fundamentally to give flexibility in how the Barbarian structures their Shout.
Arsenal Weapons need Expertise changed and made more interesting. Weapon swapping is cumbersome and just kind of stupid.