UPDATE: Sorry ! Wrong project ! I've created a new one in Spigot !

Most of popular programming language have a event system.

The main difference with Java was that until Java 8, an event system is based on classes : the developer has to register a Listener wich contains one or more methods called when an event is created. 

With the introduction of Java 8's Functions (Consumers, Functions and Lambdas), it resolve this gap.

Currently, @EventHandler in Listener classes marks methods to be called. By using Java 8's consumers, it resolves 

I would introduce an new way to consume an event :

pluginManager.registerEvent(PlayerJoinEvent.class, (event) -> {
    event.getPlayer().sendMessage("Hello !");
}, priority plugin, ignoreCancelled);

This feature should not replace the current system for compatibility reasons and for code organisation reasons.

Another way to use this feature can be :

pluginManager.registerEvent(PlayerJoinEvent.class, MyClass.PLAYER_JOIN_CONSUMER, priority, plugin, ignoreCancelled);

public class MyClass {
    public static final Consumer<PlayerJoinEvent> PLAYER_JOIN_CONSUMER = (event) -> event.getPlayer().sendMessage("Hello !");
}

Tell me what do you think.