When I talk about stealing ideas from any source, some may seem odder than others. How about Marvel Comic's Civil War for example?
If you look at the 5th Edition D&D Player's Handbook, you've got the following classes:
So how many of those classes cast spells or use some type of magic? How easy would it be to incorporate the idea of 'registration' for anyone who could cast magic? Even if it's just limited to a portion of the setting, it could create complications with most parties.
Imagine in Waterdeep you are automatically tagged and put into a school and have to work for the city.
Imagine in Cormyr you HAVE to be in the War Wizards.
In some ways, the settings often work something similar if at a reduced structure into their settings. But when you push things to an extreme level? They can take on different shades, different meanings.
It can also provide automatic breaks for the campaign. Imagine that it's not ALL magic using classes, just those that are arcane. All of the sudden you're sorcerer and warlock who didn't have to study for their magic, outside of the usual bits, now have to deal with witch hunters from all over the campaign setting. Now they have to deal with clerical spellcasters armed with dispel magic scrolls and a great knowledge of arcane magic.
Imagine the competition so fierce that warlock patrons are being killed off in the campaign by the deities of the setting, forcing people who still wish to be spellcasters to turn to deities for their power.
Pushing the ideas further, imagine groups that were once considered 'good' working on these terms. The 'Harpers' all of the sudden becomes a group that advocates for all mages to be registered and trained specifically so that they don't do any wrongs and that they have to be kept tabs on at all times. They point out the 'rogue' wizards of places like Zhentil Keep and Thay as a perfect reason why these laws must be passed and other countries, like Cormyr, fully agree, on the insistence that while in their country, these wizards work for both the Harpers and the country.
Still further, and you can see anti-magic zones like those created during the Time of Troubles becoming hot spots where the martial classes would gather to plot their works against the wizards of the world.
Still further and you can see countries using mage sniffing demons, hounds, or other entities that could sense and eat/countermagic.
This might lead to mages that don't rely 100% on their magic, mages that are multi-classed or are in hiding by claiming to be clerics or druids.
Comics can be a fun way to see how plot lines and ideas play out. Don't hesitate to steal from them.