Here are my thoughts on who the hooded man or woman for that matter.
1. Alex Luthor from Earth 3. The pre New 52 was mostly a hero who helped the Justice League defeat the CSA. That explains why the ropes and drugs are the only thing that needs to used. Also the CSA keeps saying that they need the prisoner to help defeat the Entity.
2. Dick Grayson Earth 3 seeking revenge for Owlman's murder of his parents. This one is a long shot.
3. As the article in Newsarama states it could be the New 52 Monitor. I thik this one is a long shot as well.
1 Joker of earth 3. Very longshot because he doesn't have power to fight anyone with real super powers, let alone an Earth destroying being.
No matter who this person is, they have to be either smarter or powerful enough to defeat the evil that is now on Earth One.
Here are my thoughts on Being the CSA could not defeat. In other words something more evil and powerful than they are.
1. Darkseid. He is probably the most powerful evil being in the DC Universe, and he has the ability to hop across dimensions.
2. The Anti Monitor. He too is powerful, and potentially more powerful than Darkseid.
3. A Sun Eater. Although powerful enough to destroy planets, probably not able to cross dimensions.
4. Someone new we have yet to see in the New 52.
Why it must be Darkseid
1. His forces are now attacking Earth 2.
2. He, as was pointed out in the Newsarama has been dimension hoping since his battle with the Justice League.
Why it might be the Anti-Monitor
1. the bolt that appears at the end of Forever Evil #5 looks like the ones DC used in Crisis on infinite Earths.
2. The prisoner is druged. The Monitor could be druged and subdued in such a way.
Those are my musings on the subject.
So until next time
Go Forth and Do Well