A lot of players would like to customize the inside of the house. We can do it to some extent with paintings ("wallpaper") and paving the floor inside. However, for best customization option (and easiest time to convert house to and from a "look"), I think an item that allows to make house modifications is needed.
For example, once you have metal you can craft e.g. a Tinkerer's toolbox, allowing you to swap walls, staircase, and flooring materials for a token material cost (some fraction of floor area for walls and floor, 3 per staircase, or 1/3 of that price for metal trim).
If you want to split it by tier, you can have Carpenters Toolbox (crafter with soft metal bar, can only use wood varmat for internal looks), Mason's toolbox (hard metal crafting, wood and stone), and Expert toolbox (steel crafting, allows wood, stone, metal interiors, perhaps even gems/glass/bone). And that still allows you to re-pave parts of the home in other material if you really need it.
You could also require additional items to modify the look and feel of the material. Wood blocks give outer wood texture. Wood planks - inner wood. You can spend some wood polish for a smoother less textured look. Adding a touch of cave slime modifies stone from round boulders to a more brick-like squared cobblestones. Wax makes metal "shiny". etc. all options you can experiment with in a much more contained environment of "inside".
Other topics suggested different paving options, which would be extremely welcome on the inside, and new look for brickwall, which perhaps we can allow to customize with same Mason's toolbox used for internal work. Variable material here would be appreciated so much, matching your now-stone brickwall to your well to your stone mansion.
Think of it like a Sketchbook page applied to swap a material but where you couldn't before.