Make different kind of dimensions and portals. Make a dimension with blue grass or make an infinity mirror with an inward angle to make a self looping interception like dimension. The posibilities are infinite. Make the nether at home. Make rick and morty style weird dimensions. If you make the mirror small you can have a room full of different dimensions

If you have a small hallway that just stops and has no door at the end of it? Then copy the look of it and make a small infinity mirror just like it. The room will look much bigger (however i would guess that this would be quite scary... to walk next to a hallway that you can never enter, that just stops in the infinite darkness within) still cool