Okay, it's just a theory, but I'm really confident in it! I'm going to detail it with pictures because I love visual aids in my discussions! Now, Allons-y!
We know the Cyber-Warriors want to make a siege on Gallifrey. But why? With their extensive knowledge, they must know Gallifrey is decimated, with no Time Lords left to convert!
We also know Brendan was a baby found on a planet with human-like creatures and aged faster than them. He also survived a gunshot wound and falling from a cliff, all before having his memory-wiped.
We ALSO know Gallifrey was built on the lie of this "Timeless Child." Now, we've seen the Timeless Child, so we know it isn't Brendan, but he does seem somewhat timeless, doesn't he? What if he was an attempted experiment by the humans to try and create a Timeless Child, and immortal being who could outlive all others? But, in the end, only one proper one could be produced. So the rest settled for the next best thing: being able to change their faces, rewriting their biology every time they die to survive for thousands of years!
That's right, the Time Lords are future humans! Of course, since we've seen that the human race survives more or less as is until 100 trillion, they'd have to be descendants of colonists who left Earth, likely at the start of the Cyber-Wars, who landed on Gallifrey, once a lush and placid planet, and tried experimenting on children to try and survive in a better, more humane way than the Cybermen did. Brendan was a failed attempt at making the race immortal. The Master realized that this was why the Doctor loved humans so much: because the idea of humans and Earth are buried in the memories of every Time Lord!
The Timeless Child is likely used as some kind of power source, which is why the Lone Cyberman wants to get to Gallifrey, wanting to use it as a weapon to destroy. It would also explain why the Master was so angry at this revelation, because the High Council of Time Lords also ruined his life as a child with the Drumming. This is where the Fugitive Doctor (let's go with the Third Doctor theory) comes in.
She learned of the innocent little immortal girl the High Council of Gallifrey was keeping captive to harness her energy and went rogue from the Time Lords, trying to free it. They captured her, wiped her memories of that entire regeneration, and one of the Council was forced to transfer a regeneration of their own to her so she wouldn't notice. Then they killed her, placed him in the TARDIS in the Second Doctor's clothes, and sent him to Earth, with no memory beyond being forced to regenerate in The War Games.
So how will it all change? Well, for starters, one of our companions will die, that's a given. If not Ryan, he'll probably leave the TARDIS. Hell, they'll probably all get forced out by the Doctor trying to force them all from danger and protect them from harm! The Doctor and the Master will fight, the Master trying to utilize the Child's power because we all know he's a hypocrite. The Cybermen or the Master will gain the power of the Child, and Ashad will die, believing he's accomplished his purpose. Either way, the new wielder of the Timeless Child's power will disappear to continue this arc in a future season, much like Dalek Caan's disappearance in Evolution of the Daleks. The Doctor will now be alone, prepared to fight another Time War, among the last of the Time Lords once again.
So what does this all mean? It means Doctor Who is finished, the show as we know it is over. Nothing will ever be the same. It truly will change, and be darker and more action-packed than ever before in Season 13.
It's a lot to assume, I know. But if I'm right, it means HUGE things for the entire Whoniverse. As for Ko Shamus, he's no one, and Jack will return to meet the Doctor next season. I really hope I'm at least somewhat right, because this is definitely the direction I want the show to go in next.
Am I right? Wrong? Only Sunday will tell. But until then, tell me what you think! Share your thoughts! Theories! Comments! Questions! Corrections! Weird personal facts you should've kept personal! Let me know! Love you all! Thanks for enduring this!!!