Spoiler
Super Mario on Wii U should be the epitome of all Super Mario Bros. games. With the dawn of 1080p HD graphics, the world can be larger than ever before, and the textures can be photo-realistic. I do love a traditional cartoony Mario game, but I believe that Super Mario Wii U should be the Twilight Princess of the Mario series. Mario would appear like he did in Brawl, and the Mushroom Kingdom would appear darker than before. I'd also like to see an improvement in water physics so when Mario jumps into a lake, it makes a realistic splash. An open world game like Super Mario Sunshine would be the best way to handle it, but with a bigger world than Delfino Plaza.
Here's an example of how I want the game to be, using my own idea for the game's intro:
Mario is asleep when he suddenly hears a knock at the door. He gets up to find Toadsworth covered in rain water and panting heavily. He explains to Mario that the Koopa Army is attacking Toadstool Castle, and that he is the only hope for them. Mario immediately dashes out into his front yard, where a pipe is seen at the end of it. This area is for Mario to get used to the controls, as he can run around the entire yard, change the camera angle, and even jump onto the roof to find a secret item. The rain pounds on the swaying grass as Mario leaps into the pipe and is spat out in the middle of a war-torn field. This field lies in front of Toadstool Castle, which is about 50 feet away at the moment.
Mario begins to run, but is suddenly met by a wave of Goombas and Koopas charging at him. This is where Mario learns how to attack enemies, similar to how he does in most games. As Mario runs through the long field and finally reaches the castle, a large black beast made of dark matter bursts through the stone walls. In one of its hands is Princess Peach, and in the other hand, surprisingly, is Bowser. Apparently, Bowser was kidnapping the Princess when the monster suddenly appeared through the floor and grabbed them both. The monster is enormous, and it opens a dimensional rift which sucks it inside. The player gains control of Mario again at this time, and is left free to explore the environment (consisting of everywhere he's previously been) before leaping into the portal and beginning the adventure inside this creature's mysterious dimension.
That's an interesting idea. I'd quite like to have a realistic, darker Mario game. I wouldn't want all future Mario games to be like this though.