What single change to the HP magic system can you think of which would produce the most profound effects?

I can think of three off the bat:

1. Human transfiguration can be used to achieve eternal youth. This was the premise of Lords of Magic - in the spirit of Gilderoy Lockhart, see my published works for full details.

2. Remove all forms of instantaneous transportation or communication. Goodbye apparition, floo, and portkey. Perhaps line-of-sight apparition would still be a thing, for combat purposes, but it couldn't be used to transport yourself across the country. One of the curious things about HP canon is that it has an extremely rich set of modes of transportation, but so many of them are completely redundant. This would reverse that. The big change this would make is that it reintroduces logistics to the wizarding world and allows conflict in the form of "how are we going to get there" or "how are we going to get out" type problems.

3. No Imperius Curse. Voldemort's take-over immediately becomes a lot harder.

 

1. No memory charms, the statute of secrecy is much harder to maintain and requires wizards to be much more aware of muggles and they would have to use lots of amoral methods to keep it. Killing, bribing, blackmailing, threatening and mind controling muggles would become common place.
2. Self-Spelling wands become a thing, meaning mass producing spells instead of relying on a small number of specalists is possible and would result in a wizarding version of the industrial revolution, the possiblities are basically endless in this.
3. Magic can truly bring back the dead, even if its extremely hard and only few wizard can do it, that would change society to a extremly large degree, and youd basically have legendary wizards still walk around.

 

I kinda feel like the reverse would happen: wizards would separate themselves much more from Muggles, to the extent that there was a physical divide. Wizards would not live among Muggles at all, but rather would stick to purely magical settlements hidden behind a barrage of concealment spells. As a result, wizards would be even more ignorant of Muggles.

 

They arent just hiding themselves though, they're also responsible for hiding all the other magical creatures, plants and beings, not to mention muggleborns. That would be pretty hard without memory charms and without contact to muggles, since we know that even things like dragons occasionally appear in front of larger groups of muggles.

Though I could see a large amount of wizards living completely seperately, but with a lot of enforcers of the statute living among muggles as part of their jobs.

 

I really like this idea. Instant travel and communication will surely warp the society as we know. So, I want to swing this idea to other way. Instantaneous transportation and communication is now way easier. We now have magical archways which wizards use for seemless transition from one place to another. While districs and alleys of magical world is spread all over the world, they are one place magically. You take a one step away from Diagon Alley and now you are in the French wizarding alley. There are no different ministeries for different nations, there is only Commonwealth of Magic and Magical Britain is a just neighbourhood in it.

 

A magical world with a much higher population density would be interesting, and a natural extension of a sustained period of lower infant mortality, since that would inevitably lead to selection for wizards who have large family sizes (our demographic transition started to happen in 1850's, and we are only 7 or so generations removed from high infant mortality, which has been a pretty important feature of our evolutionary context + reliable birth control and abortion our only 3 or so generations old-- if wizards have had 20 generations of exposure to this difference, then I'd expect a big change. in average attitudes/family sizes..).

Potential setting changes:

-Hogwarts as a highly selective school for the best of the best of the UK, which would explain its small class size, akin to the Philips Academy in the US.

-wizards can expand space, so wizarding cities crammed into small villages, or towering wizarding metropolises that all but encircle a Muggle City, separated only by magic.

-while food is able to be expanded by magic, there may be some limits to this, so food production may become more industrial-level than it seems to be in HP canon.

-expansion of wizards into different habitats-- wizards under the sea, conflicting with the Mermaids, or expanding to aerial habitats.

-with expansion of population likely comes expansion of government...so a more formal and powerful form of government than exists in HP canon.

-if you want to introduce some Malthusian limits to wizarding expansion, maybe magical diseases make a comeback-- a plague that strikes those that live in crowded and space-expanded cities more than in the countryside-- a dragon pox that strikes those that live in magically expanded spaces with extra viciousness.

-Obscurials and wizarding terrorists cause more damage because population densities are higher-- akin to how a bomb in NYC is worse than a bomb in a less dense village.

 

Being pureblooded actually meant something. The series would be compeltely different if pureblooded wizards were actually provably 'better,' could make for a much darker more adult series if muggle blood really did dilute magic somehow.

Not that it would be a good change, but it would be a small one with a huge impact.

 

I was under the impression that most of the people who were helping him were not under the Imperius Curse. The curse seems to have symptoms that are possible, if not easy, to detect, meaning the takeover would be delayed every time someone found one of Voldemort's puppets- friends don't let friends be puppets for dark wizards. If you really wanted a well-behaved slave in the long term it would be better to kidnap a few of his or her relatives. Someone more creative might use a false memory charm to convince the subject that he or she has done something absolutely dreadful, and threaten to tell tales.

Really, there's a lot of magic that you would think would just cause society to immediately collapse. Off the top of my head, the worst offenders are the polyjuice potion, the Imperius, and the false memory charm. As with the Imperius, I assume there's some sort of known counter to the others, maybe false memories look weird when you play them back; maybe there's an advanced spell that undoes the polyjuice potion. The Thief's Downfall seems to serve that purpose, and that's kind of the small thing that I think would be interesting to see magnified in the larger world. What if Ministry employees had to enter the Atrium through a secret array of enchantments that would remove polyjuice potion?

The trio's raid on Umbridge to get the locket in book seven would have gone much worse than it already did; if they didn't manage to retreat immediately they would have been killed right there. Arthur Weasley couldn't have told them about the enchantments even if he had been interested in them, he just passed through them every day, just like everyone else. Even if you saw someone's polyjuice go away in the Atrium, you still might not conclude that there were secret enchantments, because the effects were temporary in the first place. People trying to sneak in would have no way of planning for a mini thief's downfall effect, and for better or worse, the Ministry would be more secure than 'anyone and everyone can just walk in and kill the Minister whenever'.

Canonically I would imagine the Thief's Downfall is something only goblins and their magical crafting can manage, but it would be interesting to see what would happen if the effect could be replicated with wands.

Added Paragraph Breaks -- Zombie