"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Most specifically, sources report that a PowerPC G5 based iMac is in the works and should represent the next iMac revision.
As with many of Apple's revisions, sources are unable to provide a specific timeframe for release, but iMacs are due for a revision. The iMac was last updated in November 2003.