#1 Novak Djokovic
1. At least one more US Open - he is too good of a hard court player to only have 1. I think it would taint his record more to only have 1 US Open title while winning Roland Garros even if that means a career slam. Of course both would be nice.
2. Roland Garros - career slam obviously. Should be able to get one or more as Nadal declines further with injury and age before he declines and other players rise. But winning it might be a double edged sword. He might lose motivation to do much more, especially with family in tow.
3. More year end #1 and weeks at #1.
4. Complete set of Masters is in reach in Cinci.
#2 Roger Federer
What's left? It's almost all gravy.
1. Davis Cup championship - ok, but it's a team achievement, not individual, but he and Switzerland have never won, so definitely a worthy goal, and it's in reach this year.
2. Reaching Number 1 yet again as the oldest player would be great for him, not so good for others in other generations.
3. More slams - Another Wimbledon puts him alone at the top in history. Another US Open or Australian Open puts him alone in the Open Era. Any major won raises his already record number.
4. More titles - at 82 and counting, his next target is Lendl at 94, doable if he can maintain somewhere near his current level for 2 or 3 years or just plays longer like other greats of the past. If he can stay in the top 8, who knows how long he will play and how many titles he can win?
#3 Rafael Nadal
1. Play a full season without injury, sadly may not be realistic anymore.
2. Win the World Tour Finals. It is a major blot on the record of one of the greatest players in the sport, where he has failed against the other top 7 players, but requires #1 factor of health to be realistic. He must be healthy enough to play and compete and by the end of the year. It's also probably his worst conditions for him to play in, but he's reached 2 finals, so with some luck it could happen, and I think given his overall stature, should happen, but he still needs some good luck.
3. Win another Australian Open? - might be nice for the double career slam, but the tournament has been very costly to him in terms of injuries and defeats. It might be time to let it go as other greats did in the past, and just start his season on clay. I think if he puts in a big enough effort to win it, it will cost him Roland Garros, just as it did in 2009. Of course that may be acceptable to him, I don't know how highly he values the tournament. I've recently heard him talk about his bad luck there, so I don't know.
4. Win more slams - he's getting closer to Federer, but he is getting older and playing less due to injury, etc. His window on catching him will be closing, especially as younger players start to reach their peak, or if Federer can somehow eke out 1 or 2 more.
5. Win the most clay titles in the Open era - He will soon eclipse Vilas' record (46) on clay if he stays healthy. He has 45.
#4 Stan Wawrinka
1. Win Davis Cup - it is by far his biggest goal. Yes, it's a team event, but when you have one of the all time greats in Federer on your team, and you're a slam winner and #4 in the world you know it's up to you to play your best and not fold like a cheap tent, or get deservedly thrown under the bus. It's a tremendous amount of pressure on him and it could be a crucial point in his career. Success and he'll gain a lot of confidence. Failure and it could affect him the rest of his career.
2. Be more consistent throughout the year. Win some more masters here or there. For example, I think if he could win a couple of clay masters over some top players, especially Djokovic or Nadal, he might get the confidence needed to play very well at Roland Garros. He certainly has the game on clay to do it.
3. Win some more majors either in Melbourne, Paris, or New York. With some luck and a good run it could happen at any of those venues.
4. Do well at the World Tour Finals. A final would be extremely good for him. I believe he's beaten every player this year except Nishikori, and still has a 2-1 H2H advantage against him.