I don't think you can go more than about 5 years because it's just too hard to predict. For the men, I think the majority of the top 10 players in even 5 years will mostly be guys we haven't heard of, or guys who we might have heard of but are not yet highly ranked players. Del Potro and Cilic are two who could be up there.
I also think Djokovic stays up there. I agree somewhat with the Roddick-Djokovic comparison in terms of being consistently good, except I think Djokovic will have about 4 slams or so when he's done. Two differences: 1) the Federer obstacle will be removed soon, at least as a dominant player, and 2) Djokovic is mentally stronger compared to Roddick at the current point of his career, and I think that will be the difference in some big matches in the next few years. Murray is a wild card - I think he could go either way.
I think the men's game is entering a period which will resemble the years before Federer's dominance, where many different players will win slams and the number 1 ranking will change hands regularly. But I could be wrong - maybe a guy like Del Potro takes over.
For the women:
On the women's side its really impossible to predict. SOme people are high on Wozniacki but I did not like her attitude in the US open finals. She seemed glad enough to have made it to that point.
I understand that concern, but I wouldn't worry too much about it because she's very young. Djokovic seemed to have much the same attitude at times when losing to Federer in the 2007 U.S. Open final, and went on to win the next slam. That said, I have no idea what will happen with the women, other than I think that in 2-4 years nearly all of the women winning slams will be women who have not won any yet. Williams, Williams, Clijsters, Henin, Kuznetsova all likely to not be near the top more than a few more years. Sharapova might be the only one to stick around longer, if she can stay healthy.