"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Apple Seeds Mac OS X 10.5.3 (9d19) to Developers
Apple first seeded Mac OS 10.5.3 9d10 in late-March and has been issuing regular releases to developers.
Once complete, the final Mac OS X 10.5.3 update will become available as a free update to existing Mac OS X 10.5 (Leopard) users via Software Update. No target date is set, but testing for a new version can extend over several months.