Creating New Spells
In our campaign I am treating spells as part of the universe. They do exactly what it says in the book, no more, no less. They may have flexibility, but only if listed in the book as being flexible, permitted by a class feature, or simply equivalent to casting at a lower level than the caster.
One significant group of classes that does have the ability to alter or even create spells is the Nightbane classes. If anyone wants to play one of their casters they will follow their rules. Consider however that Nightbane is an SDC setting so your character may need to spend some time and effort altering damage dealing spells.
Creating a new spell will generally be beyond the ability of a player character. In addition to the research to create a spell it also requires casting a ritual. This ritual is itself a spell of legend. The ritual is demanding and includes a specialized first casting of the spell. Gods of knowledge and magic always know this spell as well as leaders of pantheons and many other gods. Mortals rarely know this spell but might know it, if they do, they are likely to be viewed with suspicion or worse by the gods themselves.