Stories need to have believability in the magic created for characters. I don’t like books where characters suddenly realize she or he has a power, such as a character suddenly realizing she always had the power to turn her enemies into toads with just a thought at the end of a book to solve the final conflict. The story is now unbelievable, like the author suddenly realized they put their characters into a situation she couldn’t get out of so the magical wizard suddenly finds her power. I find myself asking, “Where did that just come from?” and scanning back in the book to find what I missed.
On the other hand I don’t mind characters that are growing with their power. If the hero starts with only being able to start a soft breeze and by the end of the book he can create a hurricane, that works too. Just as long as the readers get to see the character grow and change with their new found abilities.
The heroines and heroes of my favorite stories have to have limits, if they don’t why is the antagonist even a threat? I have read a few stories where the heroes or heroines are near impossible to harm or even kill and I found myself wondering, “Why don’t they just rule the world?” I mean obviously no one can even touch them. I like limits, I can relate to characters with limits and people who have to make sacrifices for the things they want. This also creates conflict which is essential to any good story.
A great way to create limits is the source for power and magic. The source needs to be believable such as a genie who can only grant three wishes or blood as the sacrifice for more power. With limits the characters have to think before they leap. Plus this gives us romantics a way to spice up our stories, what better way to say I love you then having the main character sacrifice everything for the man he loves?
Also think about instruments needed for magic such as wands or ingredients like salt or herbs. If a mage needs to have a wand or a dove’s feather to perform magic there is a limit to the power giving him or her a weakness and believability.
So create those rules, make your characters vulnerable, and keep writing!