Federer is looking great and seems to be hitting his backhand better than ever which is a key to him competing for the slams and his movement has improved yet again. What an inspiration for anyone thinking they are getting too old to compete. If in the opposite half from Novak I would anticipate that being the final in Melbourne i.e. Roger v Novak...
Writing off Nadal probably isn’t a great idea but I think it's just too early for him to be a threat at the moment and no doubt he'll be ready for the clay court season and the strong favourite for the French Open once again, perhaps robbing Novak of his 4 Grand Slams!
The only other player I see as a potential threat to the "big 4" would be Nishikori who is a fantastic early striker of the ball. If he could improve his 2nd serve just a little bit it would make a huge difference to his game. The kink he has on the take back is pretty tough to deal with under real pressure.
I've not even mentioned Wawrinka, Dimitrov or Ferrer who are in the next tier but I just can't see any of them being good enough to win in Melbourne. As the year progresses I would hope that Stan and Grigor would pose a threat in the other slams and Ferrer will always beat everyone he should beat but generally struggles against the very best.
I've not seen enough of the womens tennis recently to comment but I would hope that Heather Watson will do well after her success in the Hopman Cup where it looked like she and Andy had lots of fun. Great to see players enjoying themselves on court instead of smashing racquets! (Serena is getting very good at destroying them.)