First off, Brandon Sanderson is a far better writer than I
. He's got some laws you should follow.
Second, Never Give Something Away For Free. It's a magic System not a magic Charity. Mind, what you have to do or pay may be disproportional to what you get but you want to explain why wizards don't simply float everywhere rather than walking.
Third, keep in mind why you want a magic system. I honestly can't expand on this, the concept is too open ended.
Finally, be consistent. If X+Y=Z then it should always =Z. While you don't need to explain the rules you should have some kind of rules or it's just random events with no connection between them.
But that's just my opinion. Take with as much salt as you want.
Granted, the first idea is: how much you want magic in your setting?.
Because sometimes magic is just powers in comic book setting, other times there is mysthic about it like the force, other is stright out reallity warping.
Start there and you can work out because is about how you want magic to be show in the first place, then the details.
"My Name is Bolt, Bolt Crank and I dont care if you believe or not"For me, personally, internal consistency is the most important part of any fictional magic (or super-science) system. If you establish what it can or can't do, or how it works, stick with it. Otherwise, see above.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"A few more things that might be worth thinking about:
- How much effort you want it to involve?
- Is it quick-and-easy flicks of a wand? Sentences in a strange language? Complex, abstruse forms sketched in the air? Exhausting rituals performed over days, weeks, or more? Something else?
- What "flavour" do you want it to have? (Other than "mushroom" ;P)
- Is it elemental? Built around manipulating—or stewarding—nature? Alchemic, or otherwise reagent-based? Arcane? Logical? Something else again?
- What is the scope of its power?
- Can it create only small effects, or big ones too? Are its effects ephemeral, or lasting? Is enchantment of items a possibility? Magitek? And how do these things affect the casting, if applicable?
- Is magic safe to use, or are there... consequences? And if so to the latter, what are they?
- Do you want magic to be well-understood by the reader, or something more out-of-reach?
- As noted in the Sanderson link above, I believe, well-understood magic is easier to use for "solving conflicts". That said, little-understood magic, can be used if applied sparingly, and carries a rather different "feel" to it. See, for example, The Lord of the Rings.
Edited by ArsThaumaturgis on Feb 27th 2020 at 6:40:27 PM
My Games and Asset PacksI think the most important question to ask is simply: What do you want to use it for?
Is it to create conflict? resolve conflict? A twist to a certain real thing (IE medieval europe but with dragons), an aesthetic? A way to make cool fights? Drama?
If your characters encounter a broken bridge, is that a big deal? What about a burning house? What if they're hungry in the middle of nowhere? IF someone shows up with a sword/axe/spear/longbow/gun, is that a problem? What about a guy in armor?
Imagine the impact magic has on various essential portions of your story. Then work through the other ideas. You should be able to fairly quickly figure out just how easy magic should be, how accessible, what kinds.
As others mentioned, be consistent. Magic is often a justified handwave, so you have to make sure the reader understands this is intentional. If a magic spell instakills a guy one scene and barely hurts the next, the audience will think one got worfed, or that the survivor got deus ex'd, unless you have a pretty damn good explanation.

I decided to throw some magic into a world-building project and...now I'm stuck. I want it to be plant- or fungus-based, use spells, and take a long time to master, but what else should be thought about? What are good questions to consider when making a magic system?