I think there is not an answer to this question...yet. I built Gumshow.com a few months ago half-way expecting to update and re-brand the site for whatever niche really takes off on Gumroad. However, I don't think that this has happened yet. While our team here is going to give Gumshow a face lift and $7mil makes me feel good that Gumroad is sure to stay in the game, I'm just not sure where the explosive growth is going to come from. I think, as one commenter here said, paying some higher profile artists (musicians, etc.) to use the site could be a very effective marketing move. It's also possible that a niche market product (that uses the Gumroad API, like Gumshow) could get things really rolling.