Announce that everyone with a FC house has failed to make their interest payments and that the local housing coalition will revoke all access if they do not pay it in 24 hours. Just give no other details and watch people freak the hell out.

Or add an achievement like "Witness the epic battle between Bahamut and the Primals in the skies above Ishgard" with the tagline (if clicked in your achievement page) that reads "You totally didn't miss that, right?"

The best part is S-E will have to work very little for these kinds of ideas to work.

A well-made set of fake patch notes works well too.