It might be a bit early for things like this, but (since there are so many interesting and surprising things being done with Arduino) how about some kind of rolling community interaction and showcasing a bit like photography.SE's photo of the week, where people post interesting Arduino projects they've made or heard about, and the most upvoted every month / fortnight / week gets showcased in, say, a short blog post where the project's author or a volunteer explains why it's interesting?
Something like this done right could build traffic and activity that turns into better questions and answers, and more loyal enthusiastic users.
There might be loads of blogs and showcase sites out there that do this sort of showcasing really well already which we might not want to compete with - in which case, this question might be a good place to drop some links.
If there are a few but they're inconsistent or infrequently updated, doing something like a "project of the month" based on a continuously open place to post submissions could be a nice way to focus attention on this site as a hub, while also promoting other sites with (hopefully popular) links through.