Twilight Princess is almost the most sold Zelda of the serie, right behind Ocarina of Time. The limits of the Zelda licence seems around this bar.
But I think that Zelda Skyward Sword has a shot at outselling it or at least outselling the Wii version alone. Twilight Princess was first only for gamecube and then delayed for the launch oif the Wii. It sold well but has not benefited from the huge success of the Wii and did not become the flagship game of the console.
Skyward Sword will be the blockbuster of th Wii next year, it is not hyped now, because Nintendo is showing nothing, but they have proved that they can release a 20 millions seller with annoucing it only 6 months before its release. I think the hype will pick up when Nintendo will start showing the game.
Someone said that the more the time pass the more difficult it's too sell on Wii because you are competing with the "evergreens" and I am not sure to agree. It's true only for new buyers, not for the one who already have these games and they are a lot.
Zelda will be bought by the core Zelda and Nintendo fans, they will probably be bought by a part of a largest audience having heard of Zelda and because it will be the Nintendo most hyped game of the year. It will beat the Wind Waker, and once he will be above he will be very near the Wii version of TP. He will surely pass it.
