I’ve been contemplating how to create a broad set of rules on when you can safely create spells that affect different types of creatures in Pathfinder 1st edition. For example, enlarge person doesn’t work on dragons… would enlarge dragon be a reasonable 1st-level spell? At 1st level we have both charm animal and charm person… would charm outsider make sense too?

There are some tricky side cases that make a universal rule hard to be balanced. For example, if you could easily apply animal growth to any creature, you end up with a spell that can potentially make giants bigger than giant form II can, and at much lower level, and that is just a terror if applied to a summoner’s eidolon.

But if we follow the example of charm person to charm monster, there do seem to be a few ways to make at least a broadly applicable set of metamagic feats that can open up flexibility, just at the cost of both a feat, and a lot of extra spell levels needed.

Monstrous Spell [Metamagic]
You have learned to adapt eldritch energies normally directed at humanoids to affect a much wider range of targets.
Benefit: Monstrous Spell can only be used with spells that limit their targets to only the humanoid creature type. This ability removes the spell’s limitation of only working on humanoids. However, it does not change that some creature types may be immune to the effects of the spell–for example Monstrous Spell applied to enlarge person allows you to use the spell to enlarge a vermin, but Monstrous Spell applied to charm person does not allow you to charm vermin that are mindless and immune to mind-affecting effects.
A monstrous spell uses up a slot three levels higher than the spell’s actual level.

Unexclusive Spell [Metamagic]
You have learned to adapt eldritch energies normally directed at undead to affect a much wider range of targets.
Benefit: Unexclusive Spell can only be used with spells that limit their targets to only the undead creature type. This ability removes the spell’s limitation of only working on undead. For example, Unexclusive Spell applied to halt undead allows you to affect creatures of any type, though most nonundead targets are considered “intelligent” (excepting only those actually lacking an Intelligence score).
This feat only works on spells that specify one or more undead as their targets, not spells that interact with undead in different ways. For example, since create undead targets a corpse, rather than an undead, you cannot apply Unexclusive Spell to it in an effort to have a spell that allows you to create creatures of any type.
An unexclusive spell uses up a slot three levels higher than the spell’s actual level.

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