So, a new feature I'm planning on bringing to the improved Floor Builder in the beta versions of Baldi's Fun New School Ultimate is custom texture support. The way this is planned to work is that you'll be able to put custom textures you want to use into a "Textures" folder, and these would be loaded into the Floor Builder, which would let you use the textures for floors, walls, etc. while creating a custom floor.
Earlier today, I did some testing in a separate project, which had promising results as you can see in the development images below. I'm hoping that by adding new features like this one, it will allow players to make even more customized floors than you could before. Just think of the floor texturing possibilities!