What if, Elliot meant to choose Harley but ended up choosing Oliver (Ollie) instead?
Okay this is going to sound crazy but hear me out:
We know that Elliot created the Young Genius Program to gather smart children all over the world for innovation. Many of the children from this program did grow up to become the scientists that worked on the Bigger Body Initiative. So, what we can gather here is that Elliot did select the best group of children with high intellect and according to the notes, Harley is the smartest.
But unfortunately, Harley lacks "humility" so Elliot sent him away to "connect with people and change".
But this strikes me as odd because if you are an innovator and a proclaimed child saviour, wouldn't you do everything in your power to guide your student yourself instead of sending him away? He even knew that Harley must have a terrible home life but still send him back. Not only that, the notes during Harley's stay in the program suggest that he had some sort of favouritism towards Harley than the rest of the other students and said that he's "perfect".... Almost like a perfect test subject???
Harley is more than just a student, Elliot actually bonded with him and they spent quality time together. That's probably why Harley was so affected by this, he viewed Elliot as a father figure throughout the program and I believe Elliot did this on purpose. Elliot ended up casting Harley away because Harley is not what he's looking for and I don't think it's just because of his smartness:
Elliot is looking for the perfect child to be his "son".
From the very beginning in Chapter 1, it's been stated that the Prototype is smart. It can create some sort of a laser from an alarm clock so the Prototype must've been a smart human back when he's alive, which is why many believe he was Elliot at first because of his intelligence.
But we later got confirmation that this is not the case but Elliot is VERY much involved in the creation of the Prototype, not just by creating but also choosing WHO to be the Prototype. In the Chapter 5 cassette tape, Elliot talked to Ollie (Oliver) as the Prototype and he talked about how they could be family and tried his best to adjust Ollie to his new situation. The Prototype experiment number is 1006 and Poppy is 1007, which aligns with Elliot telling Ollie that he will be meeting Poppy soon.
In the tape, Ollie was ranting about how Elliot was not being honest to him and he talked about a previous guardian "hitting him with a hammer", suggesting that Ollie came from an abusive household before Elliot took him in.
We don't know how Ollie was back when he was a child but he did use his old identity to trick Poppy into trusting him, which suggests that he might use his old personality as well and the "Ollie" that we see in Chapter 3 & 4 is not only a cheerful kid, but also supportive and caring.
I think you guys can see where I'm going here
What if, Ollie was also from the Young Genius Program, he possessed a higher intelligence than the rest of the kids which led to Elliot gaining interest with him, just like how Elliot started to become interested in Harley. But unlike Harley, Ollie has a good personality, a personality that fits the Playtime brand (cheerful & helpful) and possibly a good companion for his to daughter, Poppy.
So, unlike with Harley, Elliot actually took action with Ollie's toxic home life and adopted him (maybe he bribed Ollie's parents and well, he is a famous billionaire) because Ollie IS PERFECT. Perfect for the experiment (his intelligence might be what kept his human memories intact even after experimentation) and a perfect brother for his daughter. Everything he does is for Poppy so he wanted everything to be perfect. Even the house that he built for her and the factory echoes this belief of a "Heaven" for a child.
Harley "shattered" this imperfection by being a not so perfect kid so he's out of the picture. He could not have a flaw in the life that he crafted for his daughter so he went for Ollie, smart and cheery. Ollie even reflected this mentality by constantly trying to build a "Heaven" for Poppy and tried to make everything perfect. In a way, Elliot might have trained Ollie to be that perfect kid but because of Ollie's trauma (and also the fact that he was EXPERIMENTED ON), he ended up twisting the "perfect family image".
This... This would change everything if it ends up being true in Chapter 6. With the Doctor returning and the Chapter likely focusing on the Prototype and Poppy's relationship, perhaps this is the revelation that we would get. That Harley was supposed to be the Prototype but he "failed" the test. I wonder how Harley will react if he finds out that all this time, the affection and the acceptance he craves from Elliot would actually lead him to becoming another pawn. That from the very beginning, being rejected is actually a good thing but his fixation towards it ends up dragging him back into all this...