Update: According to The Verifier, the iPhone 8 will feature a Smart Connector, which it will apparently use both for charging and for augmented and virtual reality features.
Apple typically introduces a technology in one device and then rolls it out across the lineup. Retina was like that with iPhone 4 and Touch ID with iPhone 5s. The Smart Connector, which debuted with the iPad Pro in the fall of 2015, attaches via a magnet and runs power, data, and ground directly from the device. It currently powers Apple's Smart Keyboard and a similar keyboard from Logitech, with more expected to follow.
Apple could certainly engineer a Smart Connector for iPhone 8, but what it would be used for is a more interesting question. Apple made a smaller Smart Keyboard for the 9.7-inch iPad Pro, but would the company make an even smaller one for iPhone 8 Plus? For iPhone 8 standard?
If not a keyboard, then what else would benefit from a not-Lightning, non-Bluetooth connection on an iPhone 8-sized device? A speaker pack? A much better camera system? Would Apple enter the modular phone business?
What about the display? More HDs? True Tones?
Update: According to investment bank Barclays (via MacRumors), Apple's rumored iPhone 7s, iPhone 7s Plus, and iPhone 8 will feature True Tone displays.
Apple hasn't updated the resolution on iPhone or iPhone Plus since 2014. It makes them less than desirable for applications like VR, and less than competitive when it comes to spec comparisons. It does make them more energy efficient, though, given they still use LCD rather than OLED panels.
There have been rumors that we could see a bump to 1920 x 1080 for the 4.7-inch model and 2560 x 1440 for the 5.5-inch model.
There have also been rumors about OLED, but right now the LED component of the LCD system is used to implement the pressure-sensitive 3D Touch system, so would Apple re-engineer that? Or would they simply skip ahead again to something like quantum dot?
According to The Korea Herald, any and all OLED versions will be curved and made of plastic, rather than the glass typically used on flat panels.
Also, while iPhone 7 got the DCI-P3 wide gamut color system of the 9.7-inch iPad Pro, it didn't get the True Tone system. True Tone employs two ambient light sensors, with four channels each, to measure the color temperate of the surrounding area and then match the display to that temperature.
If you've ever seen a picture of an iPhone or iPad where the case looks white but the screen looks yellow or blue, that's what True Tone fixes. And once you get used to it, you want it everywhere.
Again, Apple typically pilots a new technology in a new device, but eventually rolls it out across the line. Here's hoping the same holds true with True Tone and we get the incredible calibration and dynamic compensation on iPhone 8 as well.