With the Galaxy Nexus, Google ditched capacitive/physical keys in favor of an on-screen navigation bar. Since then, apart from the Nexus devices, only some Motorola and Sony handsets use on-screen buttons. While the Android community is divided on the hardware vs. on-screen navigation buttons, the latter have the distinct advantage of being fully customizable, provided you use some custom ROM.
Most custom ROMs do allow you to customize the look and functions of the navigation bar, but there is no way to customize the navigation bar on stock ROM, even if you have root access.
While you cannot add extra features to the navigation bar on stock ROM, you can definitely change the icons with a simple app and root access.
1. Download SoftKeyZ from the Play Store. The app comes bundled with more than 150 icons, but also allows you to import your own icons.
2. Once you have gone through the introduction, tap the Start SoftKeyz button and then proceed to grant root access to the app.
3. SoftKeyZ will now show you all the custom icons that comes bundled with it. You can select individual icons for the three different navigation bar buttons by simply tapping on the ones you want.
4. Once you have selected all the icons you want to use, simply tap the Apply button. At this point your phone will soft reboot.
If you have not applied the new icons, and want to revert back to the stock ones, tap the Revert button.
5. If you want to use your own custom icons, tap the Import buttons option at the top. You can then proceed to individually select all the icons you want to use. Once selected, again tap the Apply button and wait for your Android device to soft reboot.
If you want to revert back to the original icons after applying your own icons, tap the Backup option at the top and select the Apply backup option.
If you are using a custom ROM like CyanogenMod, you will need to use SoftKeyZ every time you update to a new nightly.