One thing done very well in SL is nightclubs and discos - but there is nothing for impromptu gatherings or parties. What would be really cool is the provision of a 'jukebox database' in SL - in other words some party-type tracks from say 60s 70s 80s 90s, held as lo-fi MP3s. They wouldn't take up much server space, wouldn't sound too bad, and residents could have a jukebox in their inventory to play the selected tracks they pick.
If residents paid $1 per track that would cover royalties etc. A party would then cost about 50 L$ tops. If the music was already on the servers, it could be downloaded pretty easily to the sim. And considering that most people have at least one gesture that involves at least half of a song in their inventory already, and that this is easily retrievable and playable, a jukebox should not take up much resource at all.