I hate to dwell on something that probably won't be important for 100,000 years, but I gotta.

Those dusty arms of the galaxy might harbor alot of rogue comets, just floating out in space, waiting to get in the way of Earth. They would be equivalent to non-periodic comets: they're massive, you don't see them coming, they are moving fast all the way in, you've got a few months to react, and you don't get a second chance.

Well, maybe it's a bit premature to think about it. But maybe not, for example Comet McNaught. Brightest comet in decades. Do we know it's mass, it's composition, how to stop it if it were heading straight at us? I don't think we have a plan.