quick POC of a config-sensitive plugin factory, and the config and plugin interfaces it entails. even if we don't do this, i think it's worth noting that i think we'd be much better served in general if we tried to do less constructor injection with plugins - setter injection is typically fine. the only drawback to it is that it could be set aga…

ConfigDrivenFactory.php

<?php
/**
 * @file
 * Contains Drupal\Core\Plugin\Factory\ConfigDrivenFactory.
 */

namespace Drupal\Core\Plugin\Factory;

use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
use Drupal\Component\Plugin\Factory\DefaultFactory;
use Drupal\Core\Config\Entity\ConfigStorageController;

class ConfigDrivenFactory implements ConfigDrivenFactoryInterface {

  /**
   * The object that retrieves the definitions of the plugins that this factory instantiates.
   *
   * The plugin definition includes the plugin class and possibly other
   * information necessary for proper instantiation.
   *
   * @var \Drupal\Component\Plugin\Discovery\DiscoveryInterface
   */
  protected $discovery;

  /**
   * The storage controller for the type of ConfigEntity this factory loads.
   *
   * @var \Drupal\Core\Config\Entity\ConfigStorageController
   */
  protected $storageController;

  /**
   * Constructs a ConfigDrivenFactory.
   *
   * @param DiscoveryInterface      $discovery
   *   The discovery service used to resolve plugin ids.
   * @param ConfigStorageController $storageController
   *   The storage controller for the type of ConfigEntity to load.
   */
  public function __construct(DiscoveryInterface $discovery, ConfigStorageController $storageController) {
    $this->discovery = $discovery;
    $this->storageController = $storageController;
  }

  /**
   * {@inheritdoc}
   */
  public function createInstanceFromConfig($config_id) {
    $config = $this->storageController->load($config_id);
    $plugin_id = $config->getPluginId();
    $plugin_class = DefaultFactory::getPluginClass($config->getPluginId(), $this->discovery);
    $instance = new $plugin_class($plugin_id);
    $instance->setConfig($config);
    return $instance;
  }
}

ConfigDrivenFactoryInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Plugin\Factory\ConfigDrivenFactoryInterface.
 */

namespace Drupal\Core\Plugin\Factory;

use Drupal\Core\Plugin\ConfigEntityAwarePluginInterface;

/**
 * Factory interface for plugins that use ConfigEntity for their configuration.
 */
interface ConfigDrivenFactoryInterface {
  /**
   * Returns a configured instance of a plugin.
   *
   * @param string $config_id
   *   An array of configuration relevant to the plugin instance.
   *
   * @return ConfigEntityAwarePluginInterface
   *   A fully configured plugin instance.
   */
  public function createInstanceFromConfig($config_id);
}

ConfigEntityAwarePluginInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Plugin\ConfigEntityAwarePluginInterface.
 */

namespace Drupal\Core\Plugin;

use Drupal\Core\Config\Entity\EmbeddedPluginConfigInterface;

/**
 * Interface for plugins that use ConfigEntity for their configuration storage.
 */
interface ConfigEntityAwarePluginInterface {
  /**
   * Sets the ConfigEntity for this plugin.
   *
   * @param EmbeddedPluginConfigInterface $config
   *   The ConfigEntity object.
   *
   * @return void
   */
  public function setConfig(EmbeddedPluginConfigInterface $config);

  /**
   * Returns the ConfigEntity for this plugin.
   *
   * @return EmbeddedPluginConfigInterface
   */
  public function getConfig();
}

EmbeddedPluginConfigInterface.php

<?php
/**
 * @file
 * Contains Drupal\Core\Config\Entity\EmbeddedPluginConfigInterface.
 */

namespace Drupal\Core\Config\Entity;

/**
 * Interface for ConfigEntities that are used as embedded config by plugins.
 */
interface EmbeddedPluginConfigInterface extends ConfigEntityInterface {
  /**
   * Returns the id of the plugin in which this configuration is embedded.
   *
   * @return string
   */
  public function getPluginId();

  /**
   * Sets the id of the plugin in which this configuration is embedded.
   *
   * @param string $plugin_id
   *   The id of the plugin.
   *
   * @return void
   */
  public function setPluginId($plugin_id);
}