Design for ergonomics. On regular iPhones, you have to do much less as a designer to optimize ergonomics. The iPhone X requires you to think about comfortable button placement and usability. Ergonomics is more than just tapping, but also swiping and other gestures. Lay out your UI so all actions are accessible and as comfortably usable as possible.
It’s a whole new device: Design for it. Everyone can stretch an app out to a larger screen, but just like the iPad, a fresh approach is not only welcomed but helps you stand out in the App Store. This is a great time to validate your current design. Are your approaches still valid? Is there a better solution possible? You might come to some valuable insights that you can apply to all your designs, not just the ones for the shiny new device.
If you're a developer working on iPhone X UI updates, don't miss Sebastiaan's map visualization of the device's display.