I just thought of the craziest thing.
Nemesis arrives on Earth but, for some reason, needs a body, like he wants to get his hands dirty. So, using data from the Zeniths, he creates a clone of Elisabeth Sobeck, but with Zenith-like parts and the ability to turn her arms into weapons, like incorporating Spectre weapons into her body and making them spring from anywhere, or even Spectre-like wings. Then, he transfers himself into the body and the final boss is an evil version of Aloy, a faster, tougher, stronger version of Aloy.
What do you think ?
