search for smudge tuts and try smudging out, creating bg by smudging always looks good, grab a good colorful stock that has some color that matches your render and smudge it till it is unrecognizable then add on layers of c4d, shadows and texture, and you can branch out from there with c4d smudge, grunge brushing and smudge, then there is vector brushing, vector pentooling, gradient maps. just experiment with bgs you will find something that suits u eventually no doubt.