Initially slated for late Q2 2020, rumour has it that the OnePlus Z has slipped into Q3 2020, instead. Likely arriving a few months before the OnePlus 8T series, the OnePlus Z is expected to based on the Dimensity 1000, MediaTek's flagship chipset.
https://www.notebookcheck.net/OnePlus-Z-release-slips-to-Q3-2020.463181.0.html