Much can happen between now and Roland Garros. We don't even know how the rankings will look. I've laid out various possibilities elsewhere, but I won't make any prediction till I see the draw.
That's a good point, we have no feedback on how Djokovic will do on clay this year but he should do well.
We know Nadal will have a strong game there but he's just getting back from some knee treatment and will be short on practice at least for MC.
The other part is how Fed will end up in the rankings, he has a chance for taking the #2 spot.
This can have a big effect at RG, Nadal could end up in Djokovic's half if he drops to #3.
Then Nadal would have to go through Djokovic and this would be Fed's best draw scenario with a chance to make it to the finals against Djokovic.
The tournaments leading up to RG will play a part also, Nadal needs to do well to build confidence and so does Djokovic.
Fed's recharging himself and letting the top players run through a tough schedule, it may make a difference at RG.