Going with the assumption that Chapter 5's Shadow Crystal boss won't be a new character, I believe that it'll be the Tail of Hell.
The *literal* pointed tail.
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My interpretation of the last prophecy.
"AND THEN. WHEN ALL HOPE IS LOST FOR THE TALE
THE FINAL TRAGEDY UNVEILS.
TO SAVE THE WORLDS, THERE IS ONLY ONE WAY.
TO BE CREATED ANEW, DARKNESS MUST WASH THEM AWAY.
ONLY THEN, WILL THE WORLDS BE SAVED."
My FRIEND interpretation of the Second Voice.
The Goner's discardment was deliberate, and FRIEND was completing Gaster's sentence.
FRIEND also has Chara parellels.
My Goner Gaster interpretation of the Forgotten Man.
The purple and green motif belongs to Gaster, a man Kris saw in the shelter as a kid and repressed their memories of him.
Goner Gaster's Eggs won't lead to another ending, but will rather serve as a mental exercise for Kris.
My ICE-E interpretation of the Shadow Mantle holder.
After witnessing something horrific in the shelter, Kris escaped and lost their headband, to which Nightmare stole it.
During the Sword Route, Nightmare uses FRIEND's humble service to hijack the Original Game.