Default image for OpenGraph/Twitter Cards
When I was implementing support for OpenGraph and Twitter Cards (for both images are required) I was wondering, what should I do, if no image is available for the content... I decided not to use any image in this case though. Later, however, I discovered, that in such situations Facebook and Twitter try to find an image themselves (and, actually, use any available on the page).
Recently I found, that similar plugins for WordPress suggest to choose the default image. This really can help and, therefore, should be implemented for Meta as well.