Another feat or magic item feature can be
Improve devastation
When you cast an area spell or a damage spell that require a save, you can spend a spell slot of level 5 or lower to add extra dice to the initial damage.
You add 2 dice plus 1 die per level above 1.
For example a fireball improve with the spent of a level 5 slot will deal 14d6.
8d6 for initial fireball + 6d6 for the level 5 slot.
For lasting spell like flaming sphere it need to be test.
Maybe allow only half the dice.
Improve devastation
When you cast an area spell or a damage spell that require a save, you can spend a spell slot of level 5 or lower to add extra dice to the initial damage.
You add 2 dice plus 1 die per level above 1.
For example a fireball improve with the spent of a level 5 slot will deal 14d6.
8d6 for initial fireball + 6d6 for the level 5 slot.
For lasting spell like flaming sphere it need to be test.
Maybe allow only half the dice.