I always figured that Red Mage and Dark Knight really weren't that different, seeing as they are the only two classes that can use Black Magic other than Black Mages. Mystic Knight is somewhat different, but I felt like Red Mage and Dark Knight could both grow out of that base.

Red Mage would be primarily a healer, but they can do decent damage depending on which Sword ability they have active (Mana Sword from Mystic Knight or their Job specific Healing Sword).

Dark Knight is all damage, with a handful of draining abilities and additional damage from sacrificing health.

My full description is on Google Drive here: Mystic Knight

Please feel free to comment and let me know what you think!