If I was to go by the current schedule I would say it will be tough, but it's not impossible IMO. It shows his all around ability, but I don't think it's a record that will never be matched again. If someone is good on HC they will win 10 titles both indoor and outdoor. If they are good/great on HC, then there's a fair chance they're pretty good on grass although there are exceptions, and it's hardest to win 10 titles on grass since the players only get 2 chances a year, and that's if they play both, and one is a major. If you wanted to win 10 titles on clay, and you were pretty good on the stuff, you could do what Ferrer does every year, and just avoid Nadal by going to South America early in the year.