Without Nadal, Djoko would be the overwhelming favorite at RG. Fed is too old for red clay (in 2012, he lost to Djoko both in Rome and RG and was close to losing to Delpo at RG, it will be worse this year). The one slam I don't see Djoko winning is Wimbledon. Not that it is impossible but I would still put Fed and Murray as favorites over Djoko there.
ETA: Djoko is a terrible matchup for Delpo, I don't know how on earth you imagine Delpo could beat him at USO. I disagree with that too.
Thank you for your answer. Here is some points from which I build my prediction regarding the 2013 player's form.
1) Djokovic: it was expected from him in 2012 to have a lesser season than in 2011: because of the pressure, but mostly because 2011 was so good than it is almost impossible to reproduce it. Ever. I believe Fed's 2006 is the only season which surpass Nole's 2011. Now, his 2012 was still excellent, and he finished on a strong note. I believe he will be the most consistent at the top, but I also believe that, much alike in 2012, he will be vulnerable to his main rivals if they are in a good day, in AO less than elsewhere. He could also fall in 2013 in matches he barely escaped in 2012 (Murray, Nadal at AO, Tsonga at RG).
2) Federer: it is expected from him to lose more ground. It may happens in 2013, but I don't have much clues about it. He probably won't care of the ranking to concentrate on the major titles. As you said, Fed was not impressive in Rome nor in RG. That could mean that his winning days on clay are definitively over, or that he was simply in sub-form. I believe that the perspective to play Nadal in final if he had won against Nole in semi prevented him to try really hard. Now, if Nadal isn't there, I think he would. It wasn't so long ago that he beated absolute peak Djokovic there. In the other major, Fed will be vulnerable to more players than Murray or Nole, but he can still win any of them.
3) Murray: He has always been a though rival for Djokovic, but now he has won two big tournaments, and both of them requested that he beat Djokovic. He is a real threat anywhere outside clay (well, he was good in Rome 2011). I don't see him dominating 2013, but he surely can beat Novak in AO, Wimby, or USO.
4) Del Potro: Not sure about the bad match-up. He believe he is simply, as yet, an inferior player than Djokovic. Now, the man did win a slam before his injury, and he did showed tremendous improvement in 2012. He lost 6 time to Fed before winning their last 2 matches. He did beat Nole at the Olympics, and was close at the WTF. Maybe it won't happen this year, or not at RG, but the man will win again.
Other players, while some can hurt Fed or Murray on a good day, are irrelevant to Novak in major, except if he is bad (Tsonga at RG). But I don't see him being bad often.
Let's see what happens at the Australian.
