"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
15inch iMac Production
The move is meant to make room for an upgrade to the 17-inch liquid-crystal display (LCD) monitor, they said, but more importantly, to allow Apple to reduce inventory accumulated from more positive days.
This appears to be due to decreased demand rather than an actual discontinuation of the 15" model.