I like this idea, I also like building worlds, give everyone a better handle on telling a good story. If I could, I'd like to brainstorm some ideas.

How important was magic to the world? Obviously it played a huge roll but aside from throwing the world into chaos how wide were the effects? An event like this would strike me to be akin to the effects of the black plague, killing large portions of the population. If magic was a pillar of civilization, was it more of a luxury or a necessity, such as cities that existed in places man could not build, such as floating cities in the sky or on a lake, or magic to fight back natural elements, such as intolerable cold of the north. Heck, if magic was important in ocean trading can the ships sail properly now through routes that needed the aid of magic. Twenty years later this might be dealt with by then, but just a little bit to think about for back story, lore, and how the world works.

Aside from the environmental effects, the social and political effects would also be nothing to blink at if long distance communication leaned on magic even a little, then even after twenty years there may be cities or kingdoms that no one has heard from since the Well fell. There are also the ramifications of if a Maegi were in any sort of seat of power, that civil wars could break out for thrones, titles and land.

This is all a lot to talk and thing about so I just wanted to throw it out there.

On the note of the plague, this concept reminds me of the Spellplague from the Forgotten Realms D&D setting. Might be something to look up, as well as the Spellscarred.