In addition to @TildalWave's answer, let me add a bit.
There is at least one time that we have clearly added to the original scope, including planetary sciences. There are relatively few questions on that subject yet, but that would be a good area to start increasing the number of questions, bringing in new people.
While our visit count is low in general, there is a respectable number of pages per visit. As TildalWave mentioned, we can't post specifics, but I'm comfortable saying that I'd be happy if my personal blog had half the number that this site has.
Lastly, what would we expand our site to include? There's two major tangential topics that have been discussed that we bring in, both when we were still in Area 51. While both had some support, I think it's worth mentioning that we were the last of the 3 proposed, and the first to make it to beta.
The first is Astronomy. Astronomy is about to enter private beta any time. If the site is successful, I think we should continue as two related sites, but not the same, much as Science Fiction and Fantasy has a good working relationship with Movies and TV. If the site is not successful, I would propose that we add them to the scope of our site (We are Space.SE after all, Astronomy is space too)
The second is Aerospace, as discussed here. We could add discussions of missiles, sounding rockets, aeroplanes, and other related topics. At least for me, a site talking about Airplanes isn't that interesting. Aerospace in general is a bit more. But there is a significant number of questions on topic for this site which would be questionable in a general purpose aerospace site, such as discussing the science behind specific missions to other planets. Furthermore, a general purpose aerospace site would talk about piloting aircraft, which I really don't think mixes well for a site like this. Museums often combine the two, but I know which part of those museums I spend more time in, and that's looking at the space stuff.
Bottom line is, if you want to expand the scope of the site, take a topic that is on topic, but there are few questions about it. Then ask a bunch of questions on the topic, spread over maybe 1 per day. That will help to expand the scope. We might consider astronomy merge, but only if one or the other of us fails as a site.