Currently the hook cache is an array of the format $module_name => $group. We also know that the hook system is fragile. sun suggested to allow an alternative format with Drupal\$module\Hook::foo() (and other proposals existed before that): [#2237831] As a preparational step before any such move, I suggest to change the hook cache so that it can be filled with arbitrary callbacks. For the start, these can be added with hook_module_implements_alter().