One idea that would continue based off of the addition of class specific spells in dungeons that maybe this could be taken further in that the gear drops are also associated with the class, i.e. the mastery color a chest drops is dependent on the class the chest is acquired with. This would make the class specific experience of the game more streamlined and when another class is played either gear needs to be acquired again and/or used with the “wrong” mastery attached.
Don’t personally love the idea of another abitrary restriction.
Wrong mastery? I believe every class besides necro has at least one spell of every color. And necro can conceivably make good use of their red/green spells, even though their primaries are blue/dark.
Also, there is gear that scales off of multiple colors.