Ok, first of all, I know this would take a huge ton of work to code and keep up, and I know this will probably never come to be unless I have a crap load of money to blow, but here it is: my idea for a way to play any song while on a Minecraft server.
First off, it would work much like The Hive’s block party. A plugin that communicates with a website that has a ton of songs cataloged on it. How the cataloging would work, I don’t know. Maybe work into the Spotify api. Whatever. Anyway, it would be awesome if you could type an in game command to start listening to a song. It would be cool if you could log into the site and make an account and from there also create and share playlists that could then be listened to on any server that has the plugin. It would be pretty sweet if you could link a server to an account on the website, so then that account could be the server’s DJ. Ooh, but what if the you could upload your own songs/sounds to the we website, so the server owner could make an audible announcement to anyone listening, or with api support so other coders could play songs for when their boss mob appears, or when their minigame starts, or even cooler, battle music that would start when you fight mobs or other players. With permissions support, you could make music commands part of your donator perks and it wouldn’t go against the EULA.
Random other idea: make a WorldGuard-esque region that plays a selected song when you walk in.