BlockNGInterface.php

<?php

class BlockNGInterface extends HtmlFragmentInterface, ConfigurablePluginInterface, PluginFormInterface, PluginInspectionInterface /*, CacheableInterface */ {
  public function access(AccountInterface $account);
  public function getMachineNameSuggestion();
  // public function build(); // DIE DIE DIE
}

BlockRenderableBase.php

<?php

/**
 * Trait for blocks that are dependent on render arrays.
 * 
 * AKA, "The Block Demultiplexer"
 */
trait RenderArrayBasedBlock /* implements BlockNGInterface */ {
  // Optionally override this if the block wants to have a different way
  // of getting the title
  public function getTitle() {
    $r = $this->render();
    return $r['#title'];
  }
  
  
  public function getContent() {
    $r = $this->render();
    return $r['#content'];
  }
  
  public function getMeta() {
    $r = $this->render();
    // TODO this isn't so well-established right now
    return $r['#meta'];
  }
    
  public function getAssets() {
    // a version of drupal_process_attached() that returns found assets instead
    // of adding them to the global static caches
    return drupal_collect_attached($this->render());
  }
  
  // Ensure we the render array is built only once
  final protected function render() {
    if (!isset($this->render)) {
      $this->render = $this->build();
    }
    return $this->render;
  }
  
  abstract protected function build();
}

HtmlFragmentInterface.php

<?php

interface HtmlFragmentInterface {
  public function getTitle();
  public function getContent();
  public function getMeta();
  public function getAssets();
}