I think it comes down to a choice on how he wants to proceed with his career. Does he want to continue as he has and participate and try winning everywhere (clay, hard courts, grass, indoors) as long as he is able, or does he decide to maximize his chances for longevity and winning tournaments?
He hasn't won a title on grass or hard courts since Tokyo in 2010. Of course that doesn't mean he can't win a title on those surfaces, after all he just barely lost the AO final and others, but one would have to say his chances to win
are less than elsewhere, because the competition is relatively stronger on hard courts and grass, and with a #3 ranking likely for the next few months, the odds are even tougher.
I think if he takes the first choice, I think he'll leave the tour earlier and fall short of 50 clay titles.
So, here is one strategy to maximize his chances of winning tournaments:
1. He needs to protect his physical health and fortify his psyche to continue playing and winning titles for a few years more. I think if he doesn't, he may leave tennis much sooner than later.
2. To do this, I think he has to restrict his play on hard courts as much as possible. Hard courts are the biggest danger to his longevity, his joints, knees in particular. They are also a bigger danger to his psyche and attitude toward playing longer. The more he loses, the tougher it is for him to be mentally passionate about the game. So, he should put himself in events where he maximizes his chances of winning and minimizes his chances of injury. So this means skipping hard court tournaments wherever he can, and strategically exiting those that are mandatory.
3. Then, I think he needs to practice and play on clay courts as much as possible. One option would be to play the South/Central American events as well as the European Clay events. Clay is what revives him, clay keeps his spirits up and instills confidence in himself. Not only this, but his participation in the South American events, might draw more people and encourage some of those events to remain as clay court events for now instead of converting to hard, as has been rumored. Additionally it might influence the surface choice at the new 500 Rio tournament to begin in 2014 (I don't think the surface choice has been made yet).
Using this simple strategy, I could see him reasonably winning a variety of titles on clay for the next 4 or 5 years.
1 or 2 more at Roland Garros (2000).
4 more at Monte-Carlo (1000) and Barcelona (500).
1 more in Rome. (1000)
1 or 2 in Hamburg (500)
1 or 2 in Estoril (250)
maybe others like Stuttgart, Umag
In the Americas:
2 in Acapulco (500)
2 in Buenos Aires (250)
2 in Sao Paulo (250)
1 or 2 in Rio ? (500) if it's clay
That's 14 right there even if I stay on the low side, and only 3 tourneys are 250 events.
This is one possible plan and I have no idea if he would do it, but this is all conjecture anyway.