It's highly unlikely that we will ever get a Nougat update from ZTE. Also, because of changes that began with Marshmallow, if we get root it will be systemless root where all the binaries go on the boot partition and SuperSU is installed as a user app rather than a system app. This makes it a lot easier to install an ota update because as long as you don't change any system files, all you need to do is re-flash the stock boot.img and uninstall SuperSU to go back to complete stock.
Also, if ZTE were to do this update the same way they did the Lollipop update for the Grand X Max + it wouldn't be too difficult to make a pre-rooted TWRP flashable version of the update.
That is a lot of ifs though since we don't really know if we will ever get root, TWRP, or an official Nougat update.