Obviously this is just a theory, so share your thoughts below.
Nature of the Witness:
We know that the Witness is the oldest creature we have seen so far in the game, as it predates Rhulk which predates the Hive as a species which have been alive for like eons. We know that the Witness is not the same as the Winnower, because it and the Darkness are mentioned as seperate entities on numerous occasions. Considering the fact that egregore connects thoughts across space and time, and burning it interacts with the pyramid ships, and people claim to hear whispers coming from the plants, I believe that the Witness is some sort of culmination of egregore. Its a fungus, a hive mind, which is why the Witness has so many faces in the smoke over its head. This is also corroborated by the fact that all official in-game talk about the Witness uses the pronouns "they/them/" which, while it can mean gender neutral, it can also insinuate a plurality of identities in one. By becoming "one" with the egregore, Calus became "one" with the Witness, a disciple within it's fungal network. The Witness is likely the origin of the egregore, which it releases through its smoke to spread across the universe.
Purpose of the Witness:
Has anyone ever questioned why it is named "The Witness"? We've known since the beginning that the Light gives life (takes death), but also takes memory. The Dark takes life (gives death) and gives memory.
Given that the Witness seeks to "drink the Light" and is a "Witness" to something, its easy to speculate that it witnessed something that it wants to forget. Is it really about the Final Shape? Does it really want to whittle the universe down untill nothing is left? Or is it simply trying to forget something from it's past? Perhaps being one with the egregore has made the Witness a witness of all the pain and suffering in the universe and it simply wants to forget all of it. Perhaps it is unable to kill itself because it has been spread so vastly throughout the universe? What did it see? I firmly believe it is trying to forget something, and that the "Final Shape" is just a ruse. That, or the Witness somehow witnessed the birth of the universe and wants the universe to end, and then forget it even existed? That could blend both the "witness" aspect with the "final shape" aspect.
Let me know your thoughts!