I generally find thread based on pure speculation...weird. But this is interesting.
I believe Fed will have the most, no doubt. In case they overlap for say 40 slams (10 years)+8 slams extra for Fed, assuming Murray and Novak will decline more than Fed. Roger can win around 5W,4US,4AO,1FO out of the 40 and 2W,2US out of next 8. So 18 slams around. Novak around 1W, 3US, 4AO, 2FO i.e. 10 slams. Rafa 7FO,1W,1AO,1US - 10 slams, so Murray ends up with 2W,1US,1-2AO i.e. 5 slams.
P.s. May be i am harsh on murray, but this is just speculation