One possibility: this is Tesla's chance to showcase the finished Model 3 design, and possibly to explain more about its options and features. Just what is the "obvious thing" for Autopilot on the upcoming EV? We also wouldn't rule out further Autopilot improvements for existing cars, or hardware upgrades such as longer-ranged batteries. Tesla is fond of iterative tweaks to its vehicles, so there's no guarantee that this will be an earth-shaking announcement.
Tesla product unveiling on the 17th (unexpected by most), followed by Tesla/SolarCity on the 28th— Elon Musk (@elonmusk) October 9, 2016