To start I'd like to define Egregore: its a psychic entity which can influence the thoughts of others. Additionally it is also the name of an Angle (or watcher) from the book of Enoch. Witness could be viewed as synonymous to watcher. This is rather curious given the the fungus is called Egregore and not the Witness. We know that the Witness is not the darkness/winnower but a follower and that the darkness/winnower represents death. Egregore is a fungus, and irl fungi typically feed on dead things, decomposing them back into their raw nutrients so that new life could spring forth. Something else to keep in mind is that many of the creatures under the Witness control seem dead or in the process of dying (hive and scorn) and the Witnesses main goal seem to kill of all life, which would make a lot of food for decomposers like fungi. Now we learned from a recent lore piece that burning Egregore causes it to resonate with a strange frequency which matches a frequency that the pyramid tech gives off and gives the users visions.(ironic since irl eating certain fungi cause hallucinations) Additionally we know that the Witness seems capable of controlling the pyramids with his mind. (Possibly using that same frequency) also the Witnesses thoughts are described as being very big, perhaps too big for a single entity but maybe not for millions of interconnected fungi communicating with each other like synapses in a brain. The Witness can influence and control creatures, especially those exposed to darkness. Irl there are fungi which can control hosts that have been exposed to their spores. Further more we learned from the leviathan and glykon that exposure to Egregore spores opens you up to the darkness by connecting your mind to the fungal mycelium network. Looking at the Witness, it is always showing with that strange smoke rising out of him. Well what if that isn't smoke but rather spores. (much like fungi give off) Even Looking at his shape it does resemble a fruiting body of a fungi. Google "candlesnuff fungus" and tell me that doesn't resemble the Witness. This maybe coincidence however bungie does have a history of drawing influence from past bungie games. In the Halo series we were introduced to a parasitic creature call the "flood" which had very fungal like qualities and behaviors. The more the flood consumed the greater its knowledge and strength grew and its ultimate goal was to consume all life in the galaxy. The Witness seem to want to kill all life in the galaxy in order to reveal the final shape. If the Witness is connected to the Egregore fungus, then it makes sense that his goal would be the death of everything as this would mean more food for the fungi and allow it to spread and propagate until it was the sole life form in the universe, thus becoming the final shape. One final thought; if the Witness and Egregore are 1 in the same, then this could mean that it has been around since the events of the black garden (that were revealed in the unveiling lore book) as the Egregore would have likely been there working next to the winnower decomposing the dead flowers. Additionally this would imply that the Witness knows the rules of the flower game and that the fungus being able to interconnected and form "communities" it would be allowed to live. So the Egregore/Witness could easily wipe out all life and essentially win the flower game and become the final shape.