Brand's source is fire. Therefore his resource mechanically ought to be fire, not mana. Using skills back to back rapidly ought to bring about a second phase for Brand all together. A temporary phase of entirely new abilities, enhanced abilities, movement change, strengths and clear weaknesses.
For the 5 second duration:
Weaknesses like Brand takes 50% more damage. Brand can only cast each spell once. Brand cannot revert the transformation early. Strengths like Brand deals 50 (+1.0 ap ratio) magic damage to all enemies nearby. Brand flies through the air on a ball of fire, capable of passing over walls and objects. Brand Q is replaced with a single cast flaming whip attack that silences all enemies struck for 1/1.25/1.5/1.75/2 seconds and deals 200 True Damage. Brands W now casts in the opposite 3 directions as well at the same distance as your cast in the primary direction. Brand E now allows brand a single cast wall of fire 100 wide and 6000 long that causes 100/200/300/400/500(+.5 ap ratio) damage to any enemy who passes through it and lasts 15 seconds. Brand Ultimate remains the same.