One idea I keep thinking about, but which has captured my imagination is a website that people can post to and search for songs about historical events or periods of time. Want songs about the great depression, they would be there. Songs about Pearl Harbor, they would be there. Songs about Catherine the Great’s supposed horse fetish, yes that would be there too.
Initially, it could be just a list with songs tagged by period or type (war, life, economy, culture, etc.) that would note who wrote it, who has sung it, who published it, etc. Overtime, perhaps, it would have links to the songs so people could buy/download them, get the lyrics, etc. It should allow people to comment on the song such as its historical accuracy, the quality of the rendition, etc. Voting on the song seems like a desirable feature as well. Branching out into other media could also be a possibility, but there seems to be much to do with just songs.
My guess would be that the links for people to buy the songs would provide the income stream needed to keep the site profitable. It might even provide the ability for people upload their own songs for free or sell them through the site.
Of course it could become a venue for nazi or racist songs, so some reporting, categorizing or policing capability would be needed. Slander, libel or copyright infringement could also be an issue for uploaded songs as well.