Such as Note - Jukebox music note effect over the NPC head.

This is just a visual effect near NPC's that have an available Quest/Conversation(s) for a player.

This... This... This is a very good idea. I was thinking of a way to do it, but I didn't think of this.

I think you can use ProtocolLib for this. Maybe make it for also turn in and not just for showing npc with quests.

You can use this feature, the plugin Quests uses the Musical Note effect over NPC's heads when they have an active quest.

It's in? Or not yet? :)

Not yet. I've added it to the Trello board.