Guys, I woke up today with the craziest crackhead theory.
Imagine this: In Horizon: The Great Sea/what ever the next game is, the "Main Quest" section is split into two questlines: The Hephaestus line, and the Nemesis line.
The Haephestus Line is about Aloy and her friends searching for a way to apprehend Hephaestus, leading to them fixing the biosphere and ending The Derangement once and for all. The Nemesis line is about Aloy and her friends resolving the threat regarding the rogue Zenith AI, Nemesis. Both could have an interesting final boss, and perhaps you could gain an ally for one final fight after completing the other one.
Overall, the chances of this happening are low, but I would love to see it. Tell me what you think!