Paranoid Android and some other ROMs have the ability to change the device configuration to "phablet" mode, (basically the setup of the Nexus 7) and other modes, as well as to directly edit the properties of the navigation and notification bars. Additionally, some navigation bar replacement apps, like "PIE", and some gesture based launchers will allow you to remove the bar entirely. You can also have it "auto hide", put it in different places, etc. That's the best thing about on screen buttons... you can put them wherever you want, remove them, change them, add buttons, take buttons away, replace them, etc.