Dunno about 'dream' game... I guess that would imply something unlikely to happen, so maybe Half-Life 3.
I think I can whittle down the potential Wii U library to just four games, and I would still be willing to buy it for about £200... assuming these four games lived up to reasonable expectations (ie are as good as their previous iterations) I would be happy with the console if all I ever got to play was Mario Kart, F-Zero, Endless Ocean and Zelda.
There are not many games that could replace any of those for me in value... and those that could content are highly unlikely: Timesplitters, Half-Life and Rogue Squadron being the only ones I can think of.
For Wii U feature application (ie what to do with the controller) Mario Kart just needs to have different control methods available as in MKWii... not sure you could feasibly do anything fancy with the touchscreen except maybe put a map on it.... I guess in 1 player and online modes some neat stuff could be done, such as having a simple rear-view camera, or maybe special items that you can use the touchscreen for (for example if you have a green shell equipped it would show a top down map and you could swipe to throw it in any direction)... apart from a simple non-interactive map though, anything like that would have to be disabled for local multiplayer for the sake of fairness.
F-Zero again not sure about anything fancy. For me I would unlikely be playing F-Zero in any local multiplayer, but I also can't really think of anything other than simple map/rear view for the screen as the game isn't about items.
Endless Ocean could certainly benefit from neat controller features... the first thing that comes to mind is the photography side of things, which could be done using the controller with motion tracking like in one of the E3 demos... for example if you swim up to a fish and turn the camera on, you can track the fish with the controller screen as it swims around you (instead of trying to position the in-game character)
Zelda... well, I think if Nintendo got their heads in the right place and returned pointing functions to the IR sensor method, the Skyward Sword controls were beautiful (most of my complaints against the game are to do with forced tutorial and un-skippable text/cutscenes, the combat, puzzles and most of the controls worked well) So ultimately I would be very happy without the new screen controller.
Then again Zelda is a very inventive franchise and I would sure like to see what things they come up with for item use on the touchscreen... maybe the main game could be played with Wii remote, but you keep the Wii U controller by the side/on lap... normally displaying a map and perhaps a quick access item menu (ie instead of the current method of having 1-3 items assigned to buttons and having to swap around... many items could be selected on the touchscreen) then for special items some touchscreen use could be neat (though they would have to be careful to do this in puzzle areas only... no one wants to be swapping controllers in combat)
