Adding the possibility to register a Listener that listens to all Events.
For example by adding another HandlerList in the SimplePluginManager of Bukkit that is only for the abstract Event class

import org.bukkit.event.*

public class GlobalEventListener implements Listener {

  @EventHandler
  public void onEvent(Event event) {
    // Do some stuff with the event called
  }

}

This can help on certain applications that got for example a module system on their own to provide the ability to hot load certain features or remove them completely or just update them.