On March 23, 2017, Apple has published a patent titled “Electronic Accessory Device.” The device is basically a laptop dock for a cell phone that can be inserted into the trackpad area. Cell phone would provide computing power, graphics, memory and storage for the laptop dock with a touchscreen display. The iPhone would then be used as the trackpad. The laptop dock would include batteries.
The patent also describes similar arrangement for an iPad. The iPad plugs in as the screen rather than the touchpad.
The fact that Apple published this patent does not mean that they are actively developing this product. Nevertheless, it is interesting to see their vision to make iPhone and iPad more laptop-like.