So basically due to magical girl overpopulation they've kind of taken over everything and as they are magical in nature conventional weapons are useless so that's out of the picture. Most of the magical girls started out alright, ready to fight evil and protect the earth from various threats but some got power and control hungry and so have shaped the world toward their will in order to 'protect' humanity. The magical girls will have some kind of agency that directs them, telling both magical girls and even the citizens what to do. A lot of villains in the past have used negative feelings and other negative energy to do their work and so the magical girl society has set in rather strict rules in society to keep evil power from collecting.

Magical Girls generally come in two types, those who are abusing their powers and position as protectors and those who truly believe in what they are doing. There are many different types of magical girls, with a wide variety of powers, skills, and fighting abilities. Generally the girls who have been magical girls the longest tend to be the most powerful while the 'younger' ones gave a ways to go but should not be underestimated.

Frankly what I'm trying to figure out is what kind of setting to put this in. A school setting could work but that could also be quite limiting. Could be an active, full time resistance but I would prefer some undercover action. Then there's the possibility of being 'normal' people with jobs or school who decide enough is enough and get some power of their own to strike back. Not really sure.