As-is, the game's implementation of music seems a little lackluster. It would be nice if mods could expand the range of tracks played at certain points in the game, with optional shuffle function. Currently the Geoscape is limited to two tracks and the Battlescape to just one!
This could also be extended to allow modders to assign custom music to specific battlefield scenarios (i.e. mission types, such as terror missions or base assaults/defenses), custom memorial themes, etc.
And yes, I realise you can just use an external player and windowed mode or whatever, but it's tedious to manually keep shuffling playlists and tracks.