Brandon Sanderson's Third Law of Magic is: expand what you already have before you add something new. He explains:
Often, the best storytelling happens when a thoughtful writer changes one or two things about what we know, then extrapolates purposefully through all of the ramifications of that change. A brilliant magic system for a book is less often one with a thousand different powers and abilities—and is more often a magic system with relatively few powers that the author has considered in depth.
In epic fantasy books, it’s not the number of powers that creates immersive and memorable worldbuilding—it’s not even the powers themselves. It’s how well they are ingrained into the society, culture, ecology, economics, and everyday lives of the people in the stories.
My new project involves a slow examination of how the magic spells available to wizards would change the world they live in. Too often D&D settings are merely a simplified medieval setting, with magic limited to a handful of wizards in their towers and mystical weapons and wondrous objects. Careful thought can extrapolate how even simple spells would change the way this world works, much less more powerful spells such as teleport and magic jar.
I'm going to start with a society based on 13th century France, and then slowly introduce one spell at a time, starting with cantrips and working my way up. As I go, I'll continuously modify my theoretical society, extrapolation and changing it to reflect the cumulative consequence of the spells available to wizards.
Why 13th level France? At that time, France was the largest and most powerful country in Europe. Unlike the disastrous 14th century, the 13th century was a time of growth, invention, and cultural flowering. With a firmly established king, an entrenched system of powerful nobles, a wealthy and extensive clergy along with its downtrodden base of peasants and serfs, France from that time serves as an ideal model to what most DM's inadvertently think of when they imagine a medieval setting.
I'm going to start with some basic questions, and then move to the spells. How prevalent would magic be? Whom would learn magic? What would their limitations be, and what role would these mages play in society?