Here are Christopher Russell's three rules for crafting a system of magic. These rules should not be conflated or confused with those of other fantasy authors, including Brandon Sanderson, whose systems precede my own.
For the purpose of this discussion, magic can refer to any set of arcane or supernatural powers. It can be broad as seen in Harry Potter, encompassing a wide variety of spells and incantations that have effects ranging from turning on a light to flying through the air. Conversely, it can be narrow, such as superheroes who only have a single defining skill like super-strength. Many people may be capable of doing the same thing, or it may be a unique ability confined to a select few. For now, I'll just be talking about what allows a magic system to take hold of a reader's imagination and never let go. Your Magic Must... 1. Be Interesting: This category is extremely subjective, and is worthy of an entire post in and of itself. Let's table it for now. 2. Be Quantifiable: Unlike the amount of enjoyment a reader derives from watching a bumbling mage cast a few flickering embers instead of a roaring fireball, the numbers behind a magic system are entirely objective. "But Chris, why are you bringing something boring like math into fantasy literature?" Ah, but it doesn't have to be literal digits. A visual comparison is perfectly acceptable. Observe the 'Force' in Star Wars. Two Jedi lift the same rock, but one, the older and more experienced, is able to raise it twice as high. From this we're able to discern that the Force is something that is better wielded with experience. What if the second, younger Jedi is winded after they finish? We would say that their connection to the Force isn't as strong, or that their lack of experience caused them to use more stamina in the process. These are observable results caused by a supernatural phenomena. This could also be called the "If X, Then Y" rule. Every action, even if it's magical, should generally have an expected reaction that it produces (similar to Newton's Second Law, which states, "each action will have an equal and opposite reaction"). Variables will affect this generalization as the story winds on, but by and large it should hold true. 3. Have a Cost: Would it be fair if an amazing swimmer like Michael Phelps never got tired or never aged? If he was in peak condition for every single international sporting event he attended for the next fifty years? No, and I'm sure the Olympic committee would get tired of him winning every single gold medal that required taking a dip in the pool from now until eternity. Magic is the same way. Whether wielding it drains the user's physical energy, requires them to burn a special fuel, must be cast with a unique wand and incantation, or causes them to break out in a particularly revolting polka dot rash, it must have some sort of drawback to maintain the story's tension. Just as most tales would be boring if the heroes never won, they would be just as bland if the protagonist waltzed through every conflict without struggling to figure out how to overcome the limits of their capabilities. Good luck coming up with your own mystical ideas, and let me know if this short discussion was interesting. If so, I'll try to do posts like this more regularly and archive them for later viewing.