EDIT August 2018: This information is wrong 
The FP2 CAN’T receive Android 7.0 Nougat officially (= with Google services) because Google (= AOSP) dropped support in Android 7.0 Nougat for the msm8974 chipset (which is the Snapdragon 800/801 chipset, which is used in the FP2)
Quote from Myself5 on XDA/Disqus:
If someone here would be able to read the Android Sourcecode, they would know why the Z3 and other msm8974 devices are no longer officially supported.
With 7.0.0, all msm8974 code was removed from AOSP. After all, what you expect is that Sony is supporting a no longer AOSP supported chipset. At what point is that a fair assumption?
Of cause its sad to hear that the Z3 got the N Previews and now is no longer supported, but guess why there was no Preview 4 or 5 either? Probably that was around the time when Google gave the 8974 chipset the boot.
After all, the article is not really well researched in my eyes, and written like its Sonys fault that the 8974 platform is no longer supported. Of cause up to some point it is. But there is also a greater matter, which is not in the hands of Sony at all.
BTW, Sonys Open Devices program will probably find a way sooner or later, its just not something the company can do officially IMO.
And another quote from an XDA article:
Since support for the Vulkan Graphics API is required for a device to pass Google’s Compatibility Test Suite (CTS), any device with a Snapdragon 800/801 will not receive official support for Android Nougat. But since Nougat runs just fine without drivers that support the Vulkan API, you can expect to see unofficial ports for many other MSM8974 devices. The level of functionality of these unofficial ports will widely vary between devices, depending on how open source friendly the device’s hardware is.
So basically a FP Open build would be possible but not one with the Google services included.