Changing Zelda from an action-adventure to an RPG is like changing X-COM: UFO Defense from a turn-based strategy to a first-person shooter. It's not like Mario, which has a whole surreal reality to exploit; the brand by itself isn't as viable as the gameplay. Zelda isn't really much more than a relatively down-to-earth fantasy world, but it's made interesting by the way the player explores it, IE its signature problem solving and exploration-driven gameplay and attention to detail.
That interaction model, founded very strongly in the player's direct interaction with the setting, is the heart and soul of the series; not that you couldn't explore the Zelda world in other ways, but the problem is finding a way that serves it equally as well as its traditional formula does, and I don't think an RPG of any kind would be the right fit. If Nintendo does something new with Zelda, I think it should be more in terms of what characters and settings they choose to explore rather than genre of game. Nobody really wants Zelda to be of a different genre, but exploring it from the perspective of a new character or putting it in a new setting other than Hyrule could be very refreshing. Making changes to Ganon's approach to conquering the land might be interesting too, like say if a huge cult started in his name and everybody but Link was part of it, forcing him to be covert and infiltrate their ranks in order to prove Ganon's non-benevolence and destroy the black gate before he has a chance to come back through it.
[img src="http://upload.wikimedia.org/wikipedia/en/b/b3/Ultima_VII_Black_Gate_box.jpg" /]
Oh. Or I could just play that.
