I have a Nexus 6 that I am currently playing with which has the "soft key" navigation buttons on the bottom of the screen. It has a triangle for the back button, a circle for the home button and a square for recent apps button.
I also have an HTC One that has the same on screen navigation but the icons are a lot better looking in my opinion. It has a curved arrow for the back button, a picture of a Home for the Home button and overlapped squares for the recent apps button.
Is it possible to change the icons on the Nexus 6 without modding/jailbreaking the phone?