I think Zelda WiiU will do the 'darker' tone that we've been waiting for to return since MM and OoT. The WiiU is HD so I know that Nintendo will be glorifying the graphics for Zelda WiiU as much as they can with lighting, textures, weather, etc etc and I'm sure the best way for them to do that is go down the creepy and intense path.
Majora's Mask was surreal and intense and some people would argue dark but it was just the sense of impending doom that made the game so powerful. Its overall tone really help made it 'feel' like a dark game rather than looking it at, to look at it i'd say it was more surreal than dark, the graphics weren't that different from OoT either, it was more so the fact that the locations were twisted and almost non-sensical.
With WiiU i agree with you that more temples may be akin to something like the shadow and spirit temples of OoT, dungeons with themes like that will really show off the lighting and HD prowess of the console, but then again, so would glades shining through trees in an open topped forest temple, or the light of magma reflecting on dull metal in a fire/mine dungeon. Whatever the case the WiiU installment of Zelda in terms of art style and graphics is going to look wonderful but if the demo is anything to go by (which it probably isn't but never mind) I think that the WiiU Zelda is going be dark and brooding much like TP, Nintendo know they need to sell a lot of consoles and know that the majority of the gaming public are fickle, those who are not loyal to Zelda or nintendo don't give a toss about story etc, they'll only pick it up if it looks good, and in that I think that nintendo are going to appeal across the board with graphics just from a selling point of view, we've also seen toon/WW Link and the cel-shading style, far too much as of late so its time for a change.
It doesn't matter how dark nintendo go, they'll get away with it, is Zelda, its a new console, voice acting, going darker than black are all things that would sell the new game, just out of fan curiosity.