Djokovic has to be the favorite.
Even playing below his 2011 level he will still rack up points.
Fed did it for years with Nadal taking most of the clay points every year but Djokovic doesn't have this problem and is doing well everywhere against everyone.
2011 Djokovic had a substantial lead on Nadal, 4,000+ points at year end.
Even if Nadal makes some good changes it won't be 4,000 points worth.
There's also DelPo coming back strong hopefully, he can beat Nadal easier than Djokovic, so this is also something to be looked at.
The wildcard is Djokovic's health and not much else can stop him.
But that means you are also saying Djokovic will defend pretty much all his points. Highly unlike given his reputation and his year-end form. Afterall, he only played a good, 7 months of the entire season, unlike maybe some other seasons, like perhaps Federer where he played well from start to finish in a couple of his seasons. Will be hard to say either way. His case will be much easier to determine after Australia.