They were all pretty surprising in their own ways. Myskina winning a slam wasn't as shocking as her winning it at RG. She was a top-5 player going into RG, but most of her best results were not on clay. I think she won only 1 (?) match on clay that year not at RG. Sharapova's was pretty shocking in that she had only beaten 1 (? again I'm not sure) top-10 player before winning Wimbledon. She had won a grass court tune-up before, but it wasn't packed full of potential Wimby champions. Sveta's was semi-shocking because her hard-court form over the summer wasn't as good as it was in the stretch of tournaments were in now. She had a good Olympics though, losing to Momo in the quarters, so it wasn't too surprising. None of the players truly came out of nowhere to win, at least IMO.
As for who will win their next major, I would say it's between Sveta and Sharapova. I say Sveta because I honestly believe she has a shot at RG this year. She's taken the past 2 champions to 8-6 in the third in their championships runs, and I feel she's getting back into form mentally and physically now. As for Maria, I think it's obvious why she would be a choice to win another major son. As for Nastya, she's a tricky one. At her best, she's definitely one of the best, but at her worst, oh man. I think she'll win another one in her time, although that's not a popular opinion around here.