I'm working with my GM to have my character creating new spells for a thematic purpose, based around water. We were talking and he had asked what to consider the differences between hexes and spells. The game has guidance on how to create new custom spells (compare against ones of the same levels, etc.) which seems valid for hexes as well, but there seems to be an unspoken difference between what spells do and what hexes are limited to doing.
Hexes differentiate themselves from spells in a few ways.
They can be cast infinitely, limited only by the number of valid targets and their saving throws. This is one of the core differences from traditional spells.
They tend to be binary, like Evil Eye gives a penalty to various defenses, and Fortune gives a benefit to allies. Others are closer to spells like Shield and Water Breathing. They either work or they don't, or provide a benefit that didn't exist before. Would a Hex that increases based on caster level, such as dealing 1d6 damage per two witch levels, be overpowered?
They can still be used into higher levels, because of their binary nature. Compared to a Level 1 spell, the Slumber Hex seems inherently weaker to the Sleep spell, without the possibility of affecting multiple targets in the same turn. Still, because it's usable more than once and there's no trade-off with spell slots, it's considered one of the most powerful hexes a witch can get. Replacing traditional spells can make a hex particularly powerful, simply because of the opened spell slot.
In addition to these considerations, what should I keep in mind in creating custom hexes? What standards should a hex be compared to to determine its effective power level, whether it would unbalance the game or not? I want to know how to separate a spell and a hex when creating new content.