Much like iOS, watchOS has Notification Center accessible via a swipe down from the top of the screen. On iOS, this is accessible anywhere, whereas on watchOS it’s only accessible via the watch face. Rather than have indicators (like iOS) on the watch face that indicate you can swipe up or down for Notification Center or Glances, Apple Watch employs a red Blackberry-esque indicator dot. This indication of unread notifications is something that iOS lacks and provides a far better solution than other platforms.
Along the lines of adopting watchOS traits, I'd also imagine we'll see the motion wallpapers that are prominently featured in Watch advertising. These are even more impressive on the Watch's OLED display, where the blacks are truly black. This type of display also allows the contents of the screen to take up the full display, rather than leaving margins in software. In my example below, thing such as the status bar have been extended to the very edges of an iPhone 6 display. This type of display doesn't seem likely in an iPhone 6S update, but it certainly seems like it'll happen in the near future.
The watch face also features complications (a term adapted from traditional watch design) which can be customized to varying degrees to display relevant at-a-glance- information. Some of my favorite complications I use on Apple Watch are weather, and the activity indicator. You can see how this could look on iOS, where Apple could add a small amount of information to the fairly sparse Lock Screen. Adding complications to iOS could also make for the possibility of customizing the camera shortcut to open another app.
While no iOS device currently ships with Force Touch, editing the complications could be as simple as a long-press much like re-arranging apps on your Home Screen. Or for devices with Force Touch, with a firm press on the display. Changing the contents of a Complication could be as easy as tapping to cycle through the various options, including third-party complications (as seen in watchOS 2).