So I have an idea for an 80s style kids adventure movie about two brothers and a sister who move to New England with their mother after their father's passing and things become vitriolic between the siblings. The girl then discovers a playset mirror locked in a secret room in the basement that is actually a gateway into another dimension molded by a being with the mind of a child, whose world resembles a large blank void until someone with an active imagination steps in and everything they think of comes to life. It gives her everything a little girl could ever want, but unknown to her it is doing everything it can to keep her from leaving. So the brothers must venture into this strange Wonderland that can be both magical and sinister and try to save their sister.
The dimension inside the mirror is basically made to resemble a world built of toys, candy, an amusement park, the world of a fairy tale, stuff like that. But when it begins to collapse as the two try to escape with the sister, it begins to look more like a melting, psychedelic blank canvas. The being inside the mirror cannot create on his own. He needs to always have a muse, someone whose imagination is greater than his. Which is why he abducts children. But eventually the children he has abducted have in turn deteriorated mentally due to becoming gaunt as a result of constantly having to create this world for the being and inevitably crumble into inhuman shells, so it's a race against time.
A main theme of the movie would be the kids coming to terms with the passing of their father, the grief they feel, and ultimately using their love for each other to repair broken hearts and to push forward.