Is inheritance between content types possible? I just read Should I use a seperate content type?, and it's got me thinking.

Say I had a site with twenty-five content types, with some of the content types sharing enough of the features of some of the other content types to make some kind of "inheritance" worthwhile: I'm thinking in terms of object-oriented PHP here. Is content type inheritance possible in Drupal 6? Is it possible in Drupal 7? Or 8? Could a module be written to make it possible?
For Drupal 7 there's the Bundle Inherit module.