A component already used in the work, but enlarged and faded/blured
Noise. Choose a color and noise it up a bit to give it texture. Blured noise which is then modified to reduce the color variance can make a very nice background
Texture. Apply one of your painting programs fancy textures to either the background layer or the whole work
Whore the filters. Use all those stupid filters to make a fancy background. Just make sure that you reduce the contrast of the background so that the forground elements stand out
Of course, none of these can compare to actually generating a unique, fitting background for the work
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