There has been an interesting (at least to me, if not to anybody else) common element in most publishers' cover designs for their classics ranges. It's the horizontal band containing the author and title text.
Penguin Classics and Penguin Modern Classics
The about-to-be-launched new look for Oxford World's Classics, and the short-lived redesign of Vintage Classics
Oneworld Classics and the newly launched Capuchin Classics
Perhaps this homogeneity (which still allows for a great variety of cover images) is the reason for Penguin Modern Classics and Vintage Classics recently changing their standard design yet again.