The design of the stock Android navigation buttons has been left untouched by Google for a while now, but that could all change this year. The instantly recognizable triangle, circle, and square shapes aren't going anywhere, but they will be different if a new rumor turns out to be right.
This reportedly comes from a source that has been reliable in the past, keep that in mind. It's basically revealed that the final version of Android N may in fact ship with the button design you can see in the image below.
According to the aforementioned source, the Home button animates when long-pressed to invoke Google Now On Tap. Apparently the various colors expand in their corresponding directions until they all "file out horizontally". This isn't very easy to imagine, and unfortunately there's no accompanying GIF to show us exactly what's going on. That said, remember there will be some animation happening.
Note that this image is a mockup made based on the information in the rumor, and not a leaked one. It's unclear if all Nexus devices will get the new buttons with the Android N update or if this will be limited to the new Nexuses coming out in 2016. Given how Android device makers like to tinker with every single aspect of the OS, don't expect to see this exact design on any smartphone or tablet that isn't a Nexus.
There's a chance that this design isn't final yet and could suffer even more modifications, and there's also a very slight possibility that it's only an exploratory move and won't be pursued. But what's more likely is that we will in fact see a new button design in Android N. Now let's get ready for the inevitable internet backlash.