Cormier is outstanding. As I've said before, he is Strikeforce's Velasquez, and IMO he is actually a bit more refined than Cain, he fights calmer and less scrappy.
Cormier's weight/physique is not an issue, it's fine. He went 3 rounds with Monson no problem. He needs that tank-like build to counter his lack of height and reach, it's fine as long as it doesn't gas him within 2 rounds all the time, and it doesn't.
Barnett has an extremely effective skillset, it's bad news for anyone, but he's not taking Cormier down and he takes too many shots. Dan will most likely knock Josh out and win this tournament.
The most natural option then will be the UFC... has he ever said whether he would fight Cain? Dan always praises him and says he trains under him, I get the impression they would not fight. That will only be a major problem if Cain is still champ when Cormier is in the mix though.