Since the very first iPhone the home button has always been an important part of the user interface. This is something that could change in the future. With the invention of new technology like 3D touch there are new methods to communicate with the screen therefore removing the home button and creating a gesture is something that could easily happen.
The button currently acts as fingerprint scanner, but in a multiple patents owned by apple there are ways to turn the screen itself into a fingerprint scanner, this would completely remove the need for the physical button itself. This patent filed in in February 2015 shows the scanner built into the screen itself;
and this patent shows how it could effect the UI;