The best thing they can do for the series is make it difficult. I expect Motion+ to turn out really good, although I do know inspite of that, some people just don't want motion controls. Regardless of that, making the game alot more difficult would be sure to attract a big portion of skeptics with the series. I really hope the super guide makes its way to Zelda Wii. That would honestly save Zelda from Nintendo's new "casual" goal.
Eiji - apparently - tried to make Twilight Princess harder than Ocarina of Time. I'm not sure if he suceeded, mostly because the boss battles were too easy.