Consider how technological innovation and scientific discoveries destroyed the Middle Ages: the knight was made obsolete, and so was richness being a buttload of land and serfs. Merchants rose up; then the serfs did, too. Industries moved and grew; new habits spread, and new forms of government, too.
Suppose now a fantasy world that has a magical revolution instead of a scientific/industrial/anti-knight pikemen revolution.
What would that do to the society?
What controversial discoveries, comparable to evolution, would there be in the theory of magic and the structure of reality it revealed? Would there be magicians unwelcome not because they were icky asocial necromancers, but just the proponents of a terrible new truth?
(“It is my understanding”, the magistrate growled, “that magicians of the Kolmogorov school are all necromancers anyway. Why, if we have souls no different from the demons that animate those filthy Orcs, why should we abstain from grave-robbing and worse?”)
(“My aunt’s not an Orc!”)
Would there be magical industries (well, depends on what that magic is like); what kind of upheavals does industrial (or just new) magic cause?
Do you need a lot of people to move silver idols around? Who pays them? Are there incentives other than the threat of a fireball? How do they relate to farmers? What are their habits, beliefs, civil rights? Will they be Pharaonic slaves, or a well-fed, safely pentacled middle class? Or, due to unavoidable seepage, regularly tentacled?
Or does a working mage need a pool of one hundred children singing creepy hymns — who better get a replacement plan in place before they hit puberty. There’s a fascinating economical system to work out, if the workers are too old at fourteen, and the workmaster isn’t a genocidal maniac.
Will there be increasing pressure to legalize patrilinear skull retention for medicinal purposes?
Are mounted knights made obsolete by single novices that can fast-cast Unhinge Equine at the foe?
(“Poor General Larkingjay. He led his squadron at the enemy, and those dastardly, unchivalrous mages… you… you have not seen the face of terror until you’ve seen cavalrymen devoured by their own horses!”)
It would seem to be a stock answer that mages do not meddle in commercial affairs; they are lofty and devoted to their studies and the like; but that’s not likely. There’s always one that finds the alchemical way is not the easiest path to quick money; and that one then starts teleporting trade caravans for big bucks. Or selling unbreakable swords. Or audiences with the dead.
(There’s an idea: mages discover the life after death, and start bringing people back; priests are not happy. Or then the priests are happy; depends on what the afterlife is like. Maybe a devoutly religious mage will persuade the benevolent god Archclaw to come and visit the local temple in person. “This Miracle Salve Personally Blessed by Archclaw!”)
Even if magical aptitude is a rare inborn gift and not a learned skill, an enterprising soul could still seek apprentices and ask them, “Blindness, incontinence and raving lunacy from being cooped up with moldy books and sulfur fumes, looking for the Knowledge Supreme… or the glamorous life of a superstar magical mercantile problem-solving prodigy? Hint: the latter may involve being a bodyguard to the Princess Bodacious. Also no dirty robes or incontinence.”
What happens to a medieval world when travel becomes easy (teleportation, portals, flying horses?), lying becomes difficult (the duke judges thru truth magic!), resurrection becomes a matter of money (“But ‘necromancer’ is such an ugly word…”), or energy becomes cheap? (“This stove channels the vapors of the Abyssal Gap. Do not open. Really. Do. Not. Open.“)
Suppose there are witches that can really make you infertile / can do distance-cast birth protection. Suppose this is not a superstitious rumor, but an actual, mundane fact. Does this lead to the magistrate hiring mages to see whodunnit, or if anything was really dun? Or do the people so afflicted just seek out a different mage and curse, figuratively speaking? And which will be a more popular prank among power-mad mage-apprentices, crowd infertility or crowd superfertility?
How do the authorities handle magical crime? Can it be detected? What or rather who do you use to stop a rapist that is invisible, works memory charms, and has magical unicorn-horn Viagra? How inevitable is a coup by the police / city guard with sufficient powers to find, stop and deal with criminals like this? (For reference: On one end, the enslaved and dehumanized damane magicians of the Wheel of Time; on the other end, the vigorously franchise-enforcing bondsmage guild of Karthain in Scott Lynch’s the Lies of Locke Lamora.)
What happens when the wealthiest woman in town wears a pointy hat, and decides to push the society in a new direction? Does she even need to decide? A lot depends on whether magic is something you need to study for (i.e. you need money), or if it is something you just have (i.e. literal empowerment of the disenfranchised!). Also, whether the magical gift is rare (every could-be-apprentice will find a master, no matter their origins) or common (no free training for nobody, unless you’re a super genius). Or suppose just redheads can learn magic; hair dyes and squibs, anyone? Or maybe just women can, and anyone might, at any moment, so treat ladies right. The possibilities for pissing into all kinds of serious business are endless!
(Or to be more proactive, “Bam! I have just cast reverse gender on you. You may start to feel some sympathy for the plight of womankind any day now.”)
(“Bam! The entire kingdom is women now. The spell duration is two weeks; I expect them to be very educational, a sequence of horrible disasters, or both.”)
(“Bam! The entire kingdom is outward clones of me now. And I look good!”)
(Or, finally: “I do not wish to be a breeder. Bam! This one baby is now three. Feed ’em, husband; I’m back to the lab.” — “But honey, what do I feed—” — “Bam!”)