pathfinder 1e – Could I use a feat to increase my breath weapon?

yes, any creature with a breath weapon (which is a special attack) is eligible for the feat Skill approach, which would add +2 to DC salvation against his breath weapon. This feat is generally meant for monsters, so GM's discretion applies if a player character wants this feat.

Prerequisite: Special attack.

Benefit: choose one of the creature's special attacks. Add +2 to the DC for all throws of salvation against the special attack the creature focuses on.

There is another feat, Draconic Paragon, but it has several prerequisites and is only available to kobolds with the racial feat Draconic Breath (which gives them a breath weapon). Draconic Paragon increases the damage and frequency of your breath weapon.

Prerequisites: Draconic appearance, Draconic breath, Draconic slip, character level 10, kobold.

Benefit: You get an additional bonus of +2 against sleep and the effects of paralysis. You can use your breathing weapon twice a day and the damage of your breathing weapon increases to 4d6 points of energy damage. Your wings of the Draconic Glide feat get stronger, which gives you a flight speed of 20 feet (average maneuverability).

There are no official Paizo feats that allow creatures to change the area of ​​their breath weapons, although there are feats such as Shape Breath Weapon from third-party editors.