"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Leaked 'iPhone 6s Plus' Images Showcase Rear Casing and 'Stronger Body'
Future Supplier noticed a slight difference between the iPhone 6s Plus and last year's model, specifically in the location of a few screws used to hold the loud speaker module. But otherwise, the website noted a pretty aesthetically similar device, which makes sense for an "S" generation upgrade for the iPhone 6 line.
Another slight difference may come in the material actually used to create the rear housing for the iPhone 6s Plus, according to a source that spoke with Future Supplier. The source noted that the backing of last year's iPhone 6 Plus was "smoother" than the new iPhone, but simultaneously the 6s Plus will be getting a "stronger" rear housing, perhaps in response to the "Bendgate" controversy.