I am going to post all my season 4/5 theories soon, I just wanted to get this in while it was fresh.

Faraday....

I think that prick is going to be suffering the same symptoms as Desmond in The Constant, he was out at sea when the Island moved through space and time, so I think he will end up on the Island, having been thrown back or forward in time, and he will become a character who is going to be helping/fixing/changing things between timelines in order to help/save or correct everything.

'If anything goes wrong, Desmond Hume will be my constant..'

He has the means to keep his mind from short circuiting, it makes sense.