A plain muslin or solid black back would be, to me, the best choice for a "plain" quilt. The Amish are the Plain Folk, after all. I use muslin/black backs almost all the time - and no one has ever com"plain"ed. What this baby needs is lots and lots of hand quilting IMHO. The blocks would appreciate it! Then again, I love to have those big spaces to hand quilt. If it isn't your thing - ask the next person.