Djokovic- I think he'll have the strongest comeback. AO will probably be to soon for him, however he'll have a great chance in the other 3. I predict a slam winning year at either RG or USO and a top 2 finish.
Wawrinka- Will probably return to his inconsistent self by going out early in many tournaments and maybe a deep run in a couple of slams. He'll decline but I still think he'll finish in the top 10
Nishikori- This is really sad to see him syruggling with so many injuries. This guy cant seem to catch a break. IF he can stay fit the entire year then he should be good enough to finish in the top 8, but injury is always never too far away and could easily fall out the top 20 which I hope doesn't happen.
Kei will have the hardest start of them all, ranking may actually drop below 20 during early weeks of the season.