OnePlus looks set to release the Bullets Wireless Z alongside the OnePlus 8 series on 14 April after the product leaked in four colours. The headphones are likely to be in the neckbud design of the Bullets Wireless and Bullets Wireless 2, even though the leaked images only show part of the wire and the four alleged colours:

Leaker Evan Blass posted the image to his new Patreon, and it was also shared by Pricebaba.
Despite the name, the Bullets Wireless Z are clearly not fully wireless given the buds are attached to wires. The neckbud design allows for a wireless connection to your device, with the headphones hanging around your neck when not in use. Previous Bullets Wireless have had magnetic backs to the buds that auto-play and -pause the audio, a great feature.
We liked both the first and second Bullets Wireless, though the second-gen upped the price from £69/$69 to £99/$99. We hope the Wireless Z stick to the same price to make them worth recommending.