>>5978262Your premise seems very flexible and easy to write onto if I'm being honest (and I'm no professional writer), here's an idea:
So, whoever the main character is you have to start with first obviously. Since there are princesses stuck in mirrors, you could either start the story with them (the princesses) NOT being in mirrors (make sure they end up in the mirrors some how & here's a random idea you can use as a stepping stone, make them go to a giant ball and some witch/wizard/criminal dude crashes it and traps nearly everyone in the mirrors except the 4 girls.)
Or, take an "already-in-mirrors-route" which could take place in a collapsing theocratic type-world because the princesses are gone and have been gone for however amount of time you wish it to be. (I suppose this depends on how important you make the princesses in their society/world)
All above aside, the possibilities are practically endless. You just have to watch some (more) movies, read more books, manga, anime, webtoons, take it all in, and especially what you like.