Souths are currently ranked forth by the bookies but that's presumably without Mitchell and Arrow. If you look at the teams ranked ahead of them, there's every chance they can surpass them.
There's Canberra in 3rd who surprised a lot of teams with their no nonsense brand of football. We've seen Ricky Stuart coach teams come up with these type of seasons in the past, most notably Cronulla in 2008 and they struggled to back it up the following season. Their hopes ride on an untried halfback from Super League and while Ricky has had success in the past with other Super League players, their playmakers have often struggled in the NRL.
Melbourne haven't been the same since Cooper Cronk left. They're still a very good well drilled side, but they're missing that extra dimension Cronk bought and it's difficult envisioning them having the type of season they did last year.
Similarly, Sydney have been knocked down a peg following the loss of Cronk and Mitchell. While they did incredibly well to win back to back, they were complacent through out the regular season and I can't see that going away.