I have no problem with adult Link, but I think that the concept of Link being young is important. Even when he was 'adult' in OoT, he was still only 16-17 at the most. I would like to see a system in the next game where every time a trigger (e.g. when a dungeon is completed or a new item is obtained) is set off, Link's sprite changes very slightly to look more like an adult. It would have to be implemented so it isn't obvious every time it changes, but players will still slowly notice Link 'growing up' as they progress through the game.
The C-Stick is a brilliant idea, simple and innovative, although I would miss the freedom of camera movement that the current system allows, maybe it would be best if it was used only when locked on to a target for combat? I do believe that something original will be added to the new Zelda though, because with every previous (Nintendo developed) Zelda game, something new that is important to the core gameplay has been added. Side scrolling for the second Zelda (I forget what its called), Darkside/Lightside for ALttP, Z-targeting and Ocarina playing with OoT, three-day aspect with MM and sailing with WW.