Yeah more customisation is usually always a good thing, would love custom wallpaper and flooring to spice up my house, especially considering I don't like the majority of them that are currently in the game. I get why they wouldn't though, because for such a simple and family-friendly game, I feel like it would be a bit daunting for a younger person (or probably even a lot of adults for that matter) to create an entire wallpaper/flooring considering how big the canvas would be. Not to mention it might take up a lot of data, especially considering they already limit the small custom designs to so many slots, although I can't really comment too much on that side of things because i'm not a developer.
Now to get into the fun side of things. If literally anything was possible to customise, then i'd love to customise one of the statues in game and color them to give them some life, because the standard grey is starting to look very bland to me now. I'd also love to be able to customise a pair of shoes using the custom design pro option, and I feel like this would be very popular as well because there are a lot of people obsessed with shoe fashion out there, me included. Even just being able to add stickers like a black tick, or a stripe here and there would take the fashion game to a whole new level for me.