OK, I make my own post now:
As I wrote in another post...
WAG:
Thomas will be human after being freed by Butters' sword. And he will become a wizard, because he would have been, if not for the Hunger awakening in his youth. That's my headcanon.
So there will be 3-4 wizards from Merlin's direct bloodline to fight Outsiders in the BAT: Ebenezar (Black staff), Harry (starborn Winter Knight Warden of Demonreach...), Thomas (Ex-Whamp redeemed, cured by Fidellaccius, wielding Amoracchius) and maybe little Maggie (wizard with RCV abilities) if she is old enough by then.
But! There is one descendant missing. There should be 5 !
It is always 5. 5 points in a pentacle for example! ...
Where is the 5th descendent?
OMG WoJ says the "English Prisoner" is not Merlin. OK, but what if it is true, he is not Merlin, because Merlin wasn't Merlin after all? Merlin was TimetravelHarry.
This is a theory that has already been discussed here.
And I admit I feel there is something there.
The prisoner told Harry in the first chapter of Skin Game that he is someone who needs to be there.
What if "Merlin" (whoever he was, doesn't matter for my theory, if he was TT Harry or not) in order to keep the prison safe had to put himself in eternal stasis to keep the prison functioning. What if without his continued presence there, the wards would eventually fail.
There was a curse in the books which took it's energy from the bloodline of the curse's creator. Harry's mother's death curse for Lord Raith. Or so it seemed in Blood Rites.
There was another evidence that the bloodlines are important: the Ramps' bloodline curse in Changes.
So what if Merlin tied the power of Demonreach's wards to his bloodline and himself? So they won't weaken over time and he realized he needed to sacrifice himself to do it. Not by his death but by his life.
And either he is Harry or Merlin isn't his name any more, because he has changed so much in all this time in stasis.
This would mean, if the prisoner is freed, the wards would come down and all the prisoners are released...
...
...
Starting the BAT.
Ah, and if that wasn't clear enough, the English prisoner would then of course be the 5th descendant. Duh. 
*** thoughts tumbling over themselves in my brain making me want to type faster but not succeeding *** The 5 wizards are necessary to control the released prisoners.
Maybe it is the other way around: maybe the Outsiders break through and the EP reveals himself to Harry, telling him, he needs to release the prisoners including himself, so that the 5 descendants can use their army of monsters against the Outsiders.
