After extensive tinkering, you've learned how to improve your Eldritch Cannons:

• A Flamethrower Eldritch Cannon can be altered to a Frost Weaver Eldritch Cannon. The cannon exhales cold in an adjacent 15-foot cone that you designate. Each creature in that area must make a Constitution saving throw against your spell save DC, taking 2d8 cold damage on a failed save or half as much damage on a successful one. If a creature fails the save, it's speed is reduced by 10 feet until the start of your next turn.
• A  Flamethrower Eldritch Cannon can be altered to an Acid Brewer Eldritch Cannon. A stream of acid emanates from you in a line 30 feet long and 5 feet wide in a direction you choose. Each creature in the line must succeed on a Dexterity saving throw or be covered in acid for a number of rounds equal to your Proficiency Bonus or until a creature uses its action to scrape or wash the acid off itself or another creature. A creature covered in the acid takes 2d4 acid damage at start of each of its turns.
• Your Force Ballista has improved range. You can target creatures and objects within 240 feet of it. In addition, your attacks ignore half cover and three-quarters cover.
• Your Force Ballista has improved accuracy. Before you make an attack with your Force Ballista, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage
• When deploying the Protector Eldritch Cannon, if you cast Absorb Elements you can, as part of the same reaction, spend extra spell slots to protect creatures inside your 10-foot protective field.
• When deploying the Protector Eldritch Cannon, you can cast spells with a range of touch on any willing target inside your 10-foot protective field.