On September 29, Google announced the release of Android 6.0 and a couple of new Nexus smartphones, developed in partnership with LG and Huawei. Besides that, Google also published its list of Nexus devices that are currently pending for an upgrade to Marshmallow flavor.
Unfortunately, some Nexus owners are left out in the cold, including those that own a Nexus 7 2012 tablet. However, some good news comes from the wonderful Android community developed on XDA Forums, which has a solution to be able to upgrade it to Android 6.0, and that is through a custom ROM made of a pure AOSP firmware image published by Google.
Pay attention, as this is an unofficial bet custom ROM and at this moment the modder of the ROM warns about using any third-party software or framework such as Xposed, because the ROM he created will be unbootable or unstable.
If you know how to flash a custom ROM on an Android device, and you are aware of the consequences that might occure during install (soft or even hard brick of the device), then you can go ahead and download the ROM. Nobody is to be blamed for the damages of your device, but yourself.
The Marshmallow ROM for the Nexus 7 2012 is downloadable at XDA. Enjoy!!