I've been taking some time away from my Legend of The Wayfarer last month to do some Advanced Fighting Fantasy stuff and real life stuff, but I've updated the system (and I'll be updating books 1-5 to fit it later) and I'm about half way through book 6.
One thing that I had an issue working out was magic. I wanted magic in my world to be present, but not affect it too much. I also did not want a situation where some people were making non-magical tools, professions or weapons obsolete.
This is a topic that people ussually ask about Dungeons and Dragons. If having a bunch of spells that can do everything a rogue and warrior of a comparable level can do, then why have rogues and warriors? If every novice cleric can cure wounds, and it only takes a little bit of experience to cure diseases and poison and not too much experience to cure death, why don't people live out their natural lifespans and nver catch a disease?
One answer is that magic is rare. Dungeon Crawl Classics has a breakdown of the percentage of the population by level and 95% of the population is 0 level (which are incapable of using magic). Of the remaining 5%, almost no one is above level 2.
That is one reason.
DCC also has a magic system where there is a huge range of effects. A spell that opens doors can have an effect that ranges from complete failure to destroying all doors in a mile radius.
However, this is difficult to do in gamebooks.
Maelstrom (and it's medieval version, Maelstrom Domesday) have a very interesting version where there are no lists of spells. Instead, you describe the effect you want, and then you have to roll to see if you know the relevant spell and then you are capable of casting it. Spells are ranked in terms of difficulty depending on how removed from reality you want to be. Trying to do something that is likely to happen is quite easy. The more improbable the effect, the harder it is to pull off.
There are some interesting comparisons here. A novice Dungeons and Dragons wizard can make a random object shine like a torch, but this would be one of the hardest possible spells in Maelstrom.
I prefer this kind of magic - sure, wizards have power over the world, but the only effects that they can produce are those that could have happened anyway. This is the way that the charms skill in the Virtual Reality books worked (at least the ones by Dave Morris). If you had charms, things would happen in your favour, which were unlikely but possible (such as skarvench having a misfire on his cannon at just the right time or your charm blocking a bullet that he fired at you).
From what I remember of The Golden Bough this is how a lot of sympathetic magic worked. Hunters would put the lead bullets in their mouths before firing them as this action would represent them putting the meat from the animal that they will shoot into their mouths and make it more likely to hit something. In this case, there are still only two possible outcomes if they fire a gun at an animal - they hit it or they don't hit it - but performing the action makes hitting it more likely.
This is the kind of effect I wanted in my game, but I did not want to affect skills (which would make everyone hypercompetent). Instead, magic in my world affects luck. In Legend of the Wayfarer, magic only allows you to reroll fate rolls, which means that a) the only outcomes you get are normally possible b) your magic could just produce the same result or a worse one c) no one would see anything supernatural; just that you are slightly luckier than others.
I have not decided how many people can use this magic - it is one of the 12 possible skills, but I haven't done a skill breakdown.
There is also another type of magic (mysticism) where the character is more aware of spirits and fae, is better at communicating with them and might be able to protect themselves from them. However, in these cases, there are no huge effects.
I have given myself the get out clause that there is more powerful magic that can do impossible things in this world, but for mortals to use it, it requires several items and long incantations and sacrifice. In short, powerful magic (such as curing a disease, teleportation or enchanting something) will only be used as a plot point in books.
I have also had the idea of 'gods' in this world who wield more powerful magic. However, some of these gods are not magical - merely rare monsters (such as dragons, sphinxs, treents etc.) and the humanoid gods with magic would have the equivalent power of a level 5 wizard or cleric from Dungeons and Dragons with some immortality thrown in (although in a sense that if you kill the mortal form, it will return some time later, meaning that you can stop these gods doing something if you kill them - they just won't stay dead). This idea was inspired by a very old article about Gandalf that is all over ther internet.
I think this would make it more interesting as it will prevent a deus ex machina.
Anyway, if you want more Legend of the Wayfarer stuff, go here.