Ask HN: I have a 19,000 sub email newsletter (think blog). What do I build next?

The newsletter is at http://dlewis.net/nik -- it's a free daily thing, sharing one pretty neat non-fiction story M-F.

I am going to keep writing it and keep growing it, but I have a pretty sizable audience now, and I want to build something to leverage that audience further. I don't have any demographic data about the audience or even any interest data, and am assuming that they enjoy non-fiction because the content is non-fiction.

I'm open to all sorts of ideas as to what I can do. Please make suggestions.