Drupal blocks exposed as CTools plugins. This is just a quick POC for some internal debate, by far not the best implementation but you'll get the idea ;).

block_plugin.info

name = block_plugin
core = 7.x

dependencies[] = ctools

block_plugin.module

<?php

/**
 * Implements hook_ctools_plugin_type().
 */
function block_plugin_ctools_plugin_type() {
  return array(
    'block' => array(
      'classes' => array(
        'handler',
      ),
    ),
  );
}

/**
 * Implements hook_ctools_plugin_directory().
 */
function block_plugin_ctools_plugin_directory($module, $plugin) {
  $directory = '';

  switch ("{$module}-{$plugin}") {
    case 'block_plugin-block':
      $directory = 'plugins/blocks';
      break;
  }

  return $directory;
}

/**
 * Function to get block plugins
 *
 * @param string $name (optional) If set, only return the requested plugin
 * @return array $plugins
 */
function block_plugin_get_block($name = NULL) {
  ctools_include('plugins');
  $plugins = ctools_get_plugins('block_plugin', 'block');

  return !empty($name) && !empty($plugins[$name])
      ? $plugins[$name]
      : $plugins;
}

/**
 * Retrieves a block plugin based upon the provided delta
 *
 * @param string $delta
 */
function block_plugin_get_block_by_delta($delta) {
  $blocks = &drupal_static(__FUNCTION__);
  if (NULL === $blocks) {
    $blocks_from_cache = cache_get(__FUNCTION__);
    if (!empty($blocks_from_cache->data)) {
      $blocks = $blocks_from_cache->data;
    }
    else {
      $blocks = array();
    }
  }

  if (!isset($blocks[$delta])) {
    foreach (block_plugin_get_block() as $plugin => $block) {
      $block_handler = new $block['handler']['class'];
      $block_delta = $block_handler->getBlockDelta();

      if (!isset($blocks[$block_delta])) {
        $blocks[$block_delta] = $plugin;
      }
    }
  }

  $block_plugin = block_plugin_get_block($plugin);
  return new $block_plugin['handler']['class'];
}

/**
 * Implements hook_block_info().
 */
function block_plugin_block_info() {
  $blocks = array();

  foreach (block_plugin_get_block() as $block) {
    $block_handler = new $block['handler']['class'];
    $blocks[$block_handler->getBlockDelta()] = $block_handler->getBlockInfo();
  }

  return $blocks;
}

/**
 * Implements hook_block_configure().
 */
function block_plugin_block_configure($delta = '') {
  $block = block_plugin_get_block_by_delta($delta);
  return $block->getConfigureForm();
}

/**
 * Implements hook_block_save().
 */
function block_plugin_block_save($delta = '', array $edit = array()) {
  $block = block_plugin_get_block_by_delta($delta);
  $block->saveConfigureForm();
}

/**
 * Implements hook_block_view().
 */
function block_plugin_block_view($delta = '') {
  $block = block_plugin_get_block_by_delta($delta);
  return $block->view();
}

testBlockPlugin.inc

<?php

/**
 * CTools plugin definition.
 */
$plugin = array(
  'title' => t('TestBlockPlugin'),
  'handler' => array(
    'class' => 'TestBlockPlugin',
  ),
);

interface BlockPluginInterface {

  public function getBlockInfo();

  public function getBlockDelta();

  public function getConfigureForm();

  public function saveConfigureForm();

  public function view();
}

abstract class BlockPluginBase implements BlockPluginInterface {

  protected $block_info = '';
  protected $block_cache = DRUPAL_CACHE_PER_PAGE;

  public function getBlockInfo() {
    return array(
      'info' => $this->block_info,
      'cache' => $this->block_cache,
    );
  }

  public function getBlockDelta() {
    return substr(md5(get_called_class()), 0, 10);
  }

  public function getConfigureForm() {
    return array();
  }

  public function saveConfigureForm() {
    if (!empty($this->getConfigureForm())) {
      //
    }
  }

  public function view() {
    return array(
      'subject' => $this->getBlockSubject(),
      'content' => $this->getBlockContent(),
    );
  }

  //

  protected function getBlockSubject() {
    return '';
  }

  protected function getBlockContent() {
    return array();
  }

}

class TestBlockPlugin extends BlockPluginBase {

  protected $block_info = 'Test plugin';

  protected function getBlockSubject() {
    return t('Test block plugin');
  }

  protected function getBlockContent() {
    return array(
      '#markup' => '<p>TestContent</p>',
    );
  }

}