What would being a mage of rage entail?
With the new description of Rage by the Canon Test, about tearing down false truths, I could see a Mage of Rage knowing faith from zealotry and understanding what is a righteous conviction and what is just fanaticism. They would likely also know literal Rage, maybe either being angry themselves or knowing others who give in to wrath easily, or simply understanding why some would get all angry about something. If you believe in the interpretation of Hope as Narrative Coherency and Rage as Narrative Contrivance, you could also say the Mage of Rage would know how the story makes no god damn sense.