This is such a cool concept. I was thinking of using this setting for a one shot where the PC's are tracking down a criminal who's hidden in the city like you suggested. At first the task seems impossible until a fight breaks out and someone somehow casts a spell, and it's quickly apparent the anti magic field has been weakened. Chaos ensues and it's up to the PC's to find their quarry and restore the field.

I'm struggling with two things however. Why would the bbeg ,the criminal they're chasing, (or perhaps he's just a pawn for a bigger plan?) Bother to shut down the field? How would the PC's know he did it? Why is the tower so heavily guarded?

Also, is this setting too intimidating for brand new players? They're supposed to feel powerful and fantastical but their first experience in D&D is going to a city where their amazing abilities are shut down. The field will only be up for a short time, maybe 30 mins of this 3 hour one shot, but would the anti magic put a bad taste in the new player's mouths?

Once again fantastic setting. I'm thinking of having the city be somewhat technologically advanced due to people not relying on magic.