Pet shadows, even just oblong/oval/circle shadows, in different sizes and shapes maybe to put under your pet to give the effect of it seeming like its actually in its environment. One of my biggest pet peeves about the background items (which i love, mostly) is that its hard to make the pet seem like its actually in the equipped background. Preferably translucent/semitransparent.
It doesnt have to be anything big, just something to make using backgrounds a little more pleasing to the eye.