I have to go for da Costa, Bottas, and Raffaele Marciello.
For some reason I just cannot see the likes of Perez (will win races), Hulkenberg, Grosjean (will win a race or two), Maldonado (already won a race), Bianchi, Calado (even though I would like to see it happen), Jazeman Jaafar (talent + Malaysian $$$), Jack Harvey, Robin Frijns (amazing talent), etc. winning a championship. Some of them will be good drivers and have some success in F1, but I think the three I listed previously have the most potential.
I would rule out Frijns, not because of talent but because of budget to debute in the first place (plus as a Dutchman, I cannot believe a Dutchman would once become world champion ;))
Da Costa seems a super talent, but only after the last part of FR3.5 last year. Not enough to see him world champion yet.
I think the next world champion might be still karting, since I believe that the current world champions will divide the cake for the next 5+ seasons.
As @craig-o said, who would have picked he last three (although Hamilton and Vettel were certainly on the radar by then)? So on that basis I’m going to pick one experienced outsider rather than all top rookies to take a Brawn style title following rule changes (and also with it being such a long shot and no-one else predicting the same, if it is right then I win major bragging rights).
I think Webber could win with one of Newey’s Red Bulls, if he didn’t have to beat Vettel to do it. I don’t see that happening and he’ll probably retire before Vettel leaves. Massa probably won’t come as close as he did again, Sutil, Maldonado, and the Toro Rosso boys will probably never be picked up by a fast enough team, I’m not sure di Resta is fast enough or Grosjean clean enough to win consistently, Hulkenberg is solid but only occasionally brilliant, and given the funding constraints winnowing out lower-level drivers, I doubt the next generation will be as good as the one we’ve got.
Rosberg could do it if a good enough car came to him: he might not be the fastest, but he’s consistant, he won’t throw away points. As long as he keeps up with Hamilton this year it’s a possiblity, so he’s on my list. Perez is already in a front-running team, so he’s in contention a few years down the road, (if he doesn’t screw up too badly this year), he’s still a little flukey but he’s not bad. For my third, Kubica. He’s apparently been wicked fast in every car he’s tested this winter, he’s had no shortage of offers in other series, and he still wants to do F1. I bet in a year or two, someone will build him a car he can drive (one-handed if necessary) and he’ll be back.