"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple Seeds Mac OS X 10.5.5 (9F9)
The newest seed lists 24 new fixes addressed in this seed with a focus on Sync Services, Mail and Addressbook.
Apple has started aggressively seeding Mac OS X builds shortly after the last release. The last version of Mac OS X was released on June 30th, 2008 and provided support for MobileMe services.