Everywhere you go, you see people walking around with Apple headphones in their ears much of the time. The hardware connected to those headphones doesn't merely send music out to the ears from the phone, it also accepts signals in the other direction to enable microphone input and a few buttons for pause/play and volume up/down. So sending other sorts of signals back doesn't require any new hardware on the phone.
There currently exist devices that take body temperature by sticking a sensor in the ear. Apple has a history of sticking all sorts of tiny sensors in their phones to enable new software features. So why not an earbud thermometer?
If they stuck a temperature sensor in the headphone it would enable these features:
(1) an plain old on-demand thermometer app. Think you might have a fever? There's an app for that! Wonder what temperature it is outside? Same app! A temperature sensor inside the phone mostly tells you how hot the battery and processor are but a temperature sensor in the earbud tells you your own temperature (when you're wearing them) or the ambient local temperature (when you take them out). Both are useful information.
(2) A background thermometer-based health check. Listen to music on headphones and the iPhone can warn you if you have a fever or are overheating during a workout.
Even without more speculative features - blood sugar or blood pressure testing - this is doable. Somebody will make this product.