They could/should change the design of Wii U to make it more different than Wii. Give it a more stylish approach and little bigger (for future redesigns!) since it doesn`t seem it will have Wii`s grey support. I think they would gain a lot from this and probably lose, in perception, if they follow their current design.
The controller should give the chance to a) store downloadable games from VC or Wii/Wii Uware on the controller itself and/or b) allow for SD card support to play those games anytime you see fit.
Let developers create apps for it as they do for smartphones.
In general, for the console, it would be great if you had freedom to choose the presentation/dashboard (developers could help create apps like templates for blogs) and let me rearrange it as i see fit (music, sound effects, colors, etc.)
The online experience either in game or just browsing the internet should be kept as simple: like a social network. I don`t have friends to add to play online. Let me browse some kind of menu so i can invite someone to play with. Make a social gaming network. And here, like HappySqurriel mentioned, the cloud service would do wonders: share pics, videos, music and... why not save files aswell? I`m stuck. I talk to a friend, he does the job for me! Happy me! :) Or i send him a video of where i am, or a pic, and he knows exactly how to do it - he can even draw something in the pic in the controller and send it like that.
Also, let me chose the online dashboard. Make it customizable - and easy to navigate it.
The games, well... keep making 2D and 3D games that people love, Nintendo. Be it Mario, Metroid, Zelda, DK. People sure love them 2D!
Honestly, i don`t have much to say about this besides one point: if you want 3rd parties to succeed, help opening up a market for them in that regard. get exclusives, make 2nd parties make shooters, racers, RPGs, etc.
Just give them a market in which they can make a business.
Also, in these days of impending doom from smartphones and tablets, advertise your great VC and "...ware" catalog so people know that on Wii U they can also get great games for smaller prices.
And please, make a Wii U Super Mario World! :D
That is an awesome idea and something a lot of games should have; the ability to take an in-game screenshot and either save it to your HDD or upload it to some image hosting site. In-game videos (something some PS3 games have had, not sure of the 360) could be uploaded to Youtube.