When I make quilts for people I always ask them about the back, and my 100% response is "can't you just put something plain on" ie a plain solid colour.
Now I hate using a solid colour on the back (did it once, never again unless it's a wallhanging) it must have some texture to the colour and preferably a small scale or tone-on-tone design, though I would use something like moda marbles, or makower spray time ranges if pushed. Usually I use batiks, but I like a lot of fabric on the extra wide sections of shops, so don't usually struggle too much for a co-ordinating print
