No Home Button
Along with the possibility of an edge-to-edge screen is the potential for a flush display. Apple took a step in this direction with the iPhone 7's digital Home button, which uses haptic feedback instead of a mechanical button. The new iPhone would simply take that technology to the next level by removing the visible button altogether. The next iPhone could be the perfect opportunity to make the change complete, given the technology is up to Apple's standards. We may very well see a new iPhone without any visible Home button.
Concept art by Imran Taylor for ConceptsiPhone.