Drupal can be used to build almost any kind of website. However I, and possibly many other people - would like to learn also something else - to have more tools under our belt. It would be a waste of time to learn something that duplicates Drupal functionalities, however those are so wide, that it is hard to find anything more to learn ;) . I was considering maybe Symfony2, as it would synergize with Drupal 8, but have you any other suggestion? It should be something that would supplement Drupal in areas where its lacking.
What other CMS/Framework would you consider the good supplement to Drupal and why?