If you write fantasy, one of the things you'll hear (probably over and over) is "magic should have a cost" (or, often, must have a cost). I saw it many times when I was looking at magic systems for my story, and I've seen it plenty of times on this forum too. While I don't necessarily disagree with the sentiment (it makes for an interesting story), it's become something that people parrot without any real comprehension, like "show don't tell", or that whole thing about dialogue tags. (Hint: for the first, it's not that important; for the second, as long as they're used in moderation, it's fine.)
What it all boils down to is "balance" - people think that magic is too powerful on its own, so they want to see a "cost" so that mages can't just up and take over the world, or deus-ex-machina their way out of everything. Of course, if your magic system is designed well enough and the story is written properly, this will never happen, but that's a topic for another post. What I want to discuss, here and now, is the concept of balance.
Which means, basically, that magic should have limitations. Not necessarily a "cost": the word "cost" brings to mind something the caster has to offer or risk to use their magic: sacrifices (blood, flesh, lives), side effects (this one's popular - pain, physical disfigurement, mental issues/brain damage, illness, etc.), death (by various means), backlash (the Wiccan's threefold rule), and on and on.
Limitations, however, are another form of "cost" that many people overlook. Time is a big one - if it takes ten minutes to cast a spell, that's a huge one - magic is severely limited in its use, if not its scope. Having to draw symbols and/or diagrams, chant words, use ingredients, make gestures or a series of movements (like a full-on dance); these are all limitations. Having items that are expended upon casting the spell (not necessarily ingredients, but actual sources of the power) is also a good one. The book Masks had an interesting concept: magic is a physical thing that bubbles up from underground like oil and can be mined or harvested; you have to have some to cast a spell, but once you do, it's gone (obviously, more power = more "magic" used up). If the caster has to make a pact with an otherworld being to gain power, that's a drawback (or a cost, either one) - only those powerful enough and/or crazy enough (depending on the being) will undergo the risk for the potential profit.
Public perception (mages are shunned, reviled, or outright outlawed) is another, though I don't recommend using it on its own unless it's a central part of the story. Similarly, you could use "time to learn magic" - it could take decades to master anything beyond the most basic spells, so very few bother (again, not a great example, but I threw it in here anyway).
A foil - some way that magic can be negated or otherwise circumvented - is also a limitation, though it's better used as a plot device. In the Wheel of Time, for example, Channelers could shield each other, cutting off your opponent from the source of their magic. There was also an herb called forkroot which, when ingested, prevented use of magic for a length of time. In the Avatar TV series, benders could be prevented from using their magic by surrounding them in elements besides their own - drop a firebender in an ice cave, suspend an earthbender in a wooden cage in the air, etc., and they're useless.
Basically, what you're looking for is something that will make your story interesting. Look at your world and your magic system - what makes sense as a limitation, given the world's history and the way magic works? What would make for a good plot and situations where your hero - assuming (s)he's a mage - will have to struggle to proceed, or may even fail outright? Answer those questions, and you're on your way to making a unique magic system.
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