In Pathfinder at least, they do have rules for spell research, and it’s easier if it’s pretty similar to a spell you already know, so “fireball but it’s 10 ft wider and does d4s” is something you could get.
Or you could use metamagic feats. Widen Spell for AoE, Elemental Spell for damage type, and other properties. Though that can get expensive.
In Pathfinder at least, they do have rules for spell research, and it’s easier if it’s pretty similar to a spell you already know, so “fireball but it’s 10 ft wider and does d4s” is something you could get.
Or you could use metamagic feats. Widen Spell for AoE, Elemental Spell for damage type, and other properties. Though that can get expensive.