I want to find out which theory about the identity of the Hybrid (as mentioned in the recent Doctor Who episode “Heaven Sent”) is the most probable to be true. It's not like there are not dozens of them online but I want to know the opinion of someone who has the knowledge to explain every aspect of it.
So, I have to begin with the "Valeyard" theory: This one is obvious, so not bulletproof, the Veil followed the Doctor in a castle with a yard. This could be a good reference as the Valeyard (pronounced vel-yard) being the next future threat to the Doctor, or even himself (a threat to the Time Lords), as stated in the Classic Who.
Next, the Cartmel Masterplan. To begin with a little history:
...Omega and Rassilon were the founding fathers of Gallifrey. They towered above the Time Lords who followed. They were demigods. [Dialogue in Silver Nemesis was] a subtle attempt to say that there was a third presence there in the shadowy days of Gallifrey's creation. In other words, the Doctor was also there. So he's more than a Time Lord. He's one of these half-glimpsed demigods.
The Hybrid could be The Other, a past(or future regeneration of the Doctor). We know that Steven Moffat is a great classic-whovian, so he might want to revive an old story. Therefore, how could he introduce this?
In the end, the origin theory. At the end of the episode, the Doctor says:
The Doctor: Go to the city. Find somebody important. Tell them, I'm back. Tell them, I know what they did, and I'm on my way. And if they ask you who I am, tell them, I came the long way round.
You can probably still hear me... so just between ourselves... you've got the prophecy wrong. The Hybrid is not half Dalek. Nothing is half Dalek. The Daleks would never allow that. The Hybrid destined to conquer Gallifrey and stand in its ruins... is me.
And we find out that the Hybrid is a combination of the greatest two warrior races. That could be the Time Lords and the humans(we reached the end of the Universe, didn't we?). This could lead also to the fact that he will fight with other Time Lords in the following episode. Other way of looking at this sentence is that Lady Me/Asildr, the latest immortal character that appeared in the show, is going to destroy Gallifrey. She already appeared in three episodes and she seems that has more to go.