There has been many suggestions about adding new battle music, but having them officially on Showdown is a huge copyright liability and allowing users to upload mp3s requires server space. This is clearly a very popular request since there are 4 pages of closed suggestions.
Why not add an option that allows users to use YouTube playlists or Soundcloud URLs; a feature that selects a random song from a off-site hosted playlist when a battle starts?
I think it's a relatively simple, satisfying solution to allow users to play new Pokémon OST (or whatever song is preferred) that isn't just caveman workaround "just use another tab" (which frankly ruins the immersion).