I think a plugin that let you specify a button as a "doorbell" would be awesome. You could use a command such as '/db add' and then right click a button. Whenever that button was pressed, the user that created the doorbell would get a message they could set with '/db msg' or a set message like "Someone's at the door!" (You get the gist). It would also be nice to be able to turn the doorbell notifications on/off from within the game, with like '/db on/off'. I just thought it'd be neat, because noteblocks are useless when you're down in a mine or far away.