Apple wants to add Face ID, an Action button, a telephoto camera, and a thinner, lighter body to future iPhone Duo models, but the first of those may not make the next one, according to Mark Gurman.
Gurman said Apple has already built iPhone Duo 2 prototypes with Face ID on the outer display. Even so, he said Apple is "running out of time" to finalize the feature for that model, so it might not arrive until the iPhone Duo 3. He gave no timeline beyond that, and he did not say whether the other three goals are tied to a particular generation.
A quick translation of the list: Face ID is Apple's face-scanning unlock system, and the Action button is a programmable physical button Apple has added to other iPhones. A telephoto camera is a lens with optical zoom for distant subjects. Thinner and lighter needs no translation.
The useful detail is where Gurman placed the uncertainty. A prototype with Face ID on the outer display exists, so the open question is finalizing the feature, not whether Apple can build it. Those are different problems, and a prototype alone does not tell you which generation of a product will ship with a feature.
Face ID, an Action button, and a telephoto lens all exist elsewhere in Apple's iPhone lineup. The work Gurman describes is therefore fitting them into the Duo, and outer-display Face ID is the one he flagged as short on time.