Just from following James on Twitter, he is one hell of an overscheduled man. I wouldn't be surprised if he's the one gone simply because they can't book him long enough. He's never on the same continent for more than a week it seems.
I'd always assumed that him being chosen late was the reason most of TGA S2's challenges were Steve and Warren, but perhaps his scheduling was more the reason, which would add weight to him being gone. Which would be a shame, he fit in very well.
The problem is that all of them currently fill a niche the show needs. Unless they bring in a proper motoring journalist (who from all previous commercial car programs are proven to be the most boring people in the world), no one floated so far from the Nine stable really fills the gap the removal of any of the current presenters would.
Steve is the real driver of the lot. TGUK has the ability to do pretty good shootouts between Clarkson and Hammond around the track. You nearly got to that with Morrison and Pizzati, removing Pizzati would mean you wouldn't have a dedicated car enthusiast on the show, and no obvious choice to replace him in that regard would improve things.
Warren brings such a unique brand of humour and is also a very strong host and interviewer. I'd also be tempted to call him the best reviewer of the lot, but I think scripts were the issue for all presenters there.
Of the floated options, Warne is useless, he sucks on the Cricket coverage, let alone trying to be intelligent on a motoring show. Anyone can cock about on TV, but the thing about Top Gear is that they have other levels of humour as well. Newman is a better option, but he is currently in the courts over road rage and speeding, perhaps not the message Nine will want to send directly.