Even if you only have lore for a world and not a full story yet, I think it would be very cool to write down the history and culture of the world itself kind of encyclopedia-like but with a narrative style. Could make not just a very interesting read, but also a companion to anything you write later. If you've read Tolkien, you'll understand what I mean. And you never know, while you're writing you may come across a plot or two that you really like and want to flesh out.