Custom Music Discs

You could make a Blank Disc by smelting any regular Music Disc.

Then, make a Disc Label by putting concrete powder of any color surrounded by 8 paper in a crafting table.

Both of the above could be dyed similarly to leather armor.

You could make a Blank Composer's Reel by putting a piece of paper into a crafting table and surrounding it with 8 noteblocks.

While holding a Blank Composer's Reel, right-click it to compose a song in a special UI, with the option to just import a MIDI file instead. When done, press Done to make an Etched Composer's Reel.

Or, you could make a Blank Echo Reel using a Blank Composer's Reel surrounded by 8 Echo Shards to import an MP3 file, either from a link or the player's computer. When done, press Done to make an Etched Echo Reel.

After that, put the Blank Music Disc and Disc Label with the Etched Composer's Reel or Etched Echo Reel into a Crafting Table to make a custom music disc.

After that, it would behave like a regular music disc, but with the custom song.

Server owners could blacklist/whitelist different websites to prevent players potentially uploading inappropriate/broken audio.