There are a lot of good project ideas in this thread. I've tried to collect all of the specific ideas. I present these here in a poll. Since there are a lot of ideas, and an unknown number of voters, I'm not sure how good of a picture we will get.
I'd prefer some kind of instant-runoff voting, but we can't do that here, so let's simulate it with multiple polls. If this initial poll doesn't show a clear winning idea, let's cut the number of ideas in half (keeping the top 50% vote-winners) and try again. We'll pare it down eventually.
The options are presented in a randomized order.