This, I know, is a somewhat open ended question but does Vuforia offer the capability to create a wallpaper app?
With it's current tracking options I'm not sure how you could identify a wall, create a mesh (of the wall or access one if Vuforia creates it) and then modify the material.
I could see the user possibly building their own wall by marking points on the ground plane (like the iPhone measure app) then entering a height to create a mesh.
or I could sample pixel colour within a range and adjust the colour to suit...
any ideas or thoughts would be appreciated. This may even be more suited to ARKit?