First of all I got invested at backrooms ever since I saw kane's pixel video about it, So now I want to read one that has someone who survives through level by level.
So my Idea is someone or many people fall through the backrooms with different surface knowledge about it. So after they have fallen through the backrooms something tells them that in order to leave the backrooms, they can need to pass all hundred floor levels or depends on how many levels you want to add. Once they have passed everything they can now make any wish and it will Come true no matter what they want be; a god, be immortal, A world moldable by you, etc.
Also the twist to make it have many floors is because SCP's are in the backrooms too, so you'll have to make a clever idea as to how your MC or group surpass each level. You can add a lot of spices to it, such as there are people who are living in it, or add special items, add Human Guards that patrol the area and kill everyone they see, etc.
So that's it Have Fun.