This could be part of the search feature. When you can't find band or song that you're searching for. You could click, "add to wish list". There could also be a feature called the petition button. Users could add artists that are not on Spotify or who don't have all their music on Spotify and then have other users add their "signatures" to the petition and then somehow by the magic of technology, it could send the petition to the artist. Sorry if this already exists. If so, then I like and support your idea.