"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Sept 9th: iTunes 8, iPhone Firmware 2.1... with Hidden Features?
Also expected is the iPhone 2.1 firmware which had been also reported with the original rumor.
One new tidbit, however, is that Ars was told that there are parts of the iPhone 2.1 firmware "that were specifically removed from developer seeds in order to keep them secret from the world." If true, that means there may be some surprise features that will be announced on September 9th.
Developers may note that Apple has ceased delivery of iPhone 2.1 firmware betas over the past few weeks, which is consistent with it having been declared Gold Master in anticipation of the upcoming release.
Apple is holding a special media event to announce their new products on Tuesday, September 9th.