The modular accessories Moto Mods were announced back in 2016 with the Moto Z and Moto Z Force smartphones. Many thoughts that the original accessories won't support the new 2017 smartphones, but as it turns out they will support even the Moto Z3 or whatever it will be called.
The company has now officially confirmed that the Moto Mods will have "three generations of support". That means there is a big chance that the successor to the Moto Z3 won't support the current Moto Mods. This also means the Moto Z3 won't feature much different design than the current Moto Z2 smartphones.