1) Better Data Management options
You should already know it’s possible to use an external device for your games, but unlike the Wii, everything is under one subcategory per each title: the core game, the save data, the update data, the DLC, etc.
There're two problems with this option:
First, it’s too slow: accessing your games' management list when your library got bigger can take some time (up to five minutes), but trying to copy or moving files can be a nightmare, where just moving an average size game like Mario Kart 8 can take up to around 15 minutes (If we compare with Bayonetta 2, that would take more than one hour!)
And second, you can’t copy certain parts of the game: What this means? In order to do a backup of your save data, you need to copy the whole game with it, and that takes a lot of space. Meanwhile you can delete single elements once copied in the device, that isn't a good option either: if you have just the save copied on your external memory, and the game in your console, copying the save data will literally overwrite the game needing to download it again.
This is lame.
2) A Multimedia player
A normal feature in other consoles is being able to play video and music files using an external device, and even the Nintendo 3DS allows you to play your own music. What about Wii U? Nothing.
It’s true it’s to watch movies on services like Hulu, Amazon Video and Netflix, or even use YouTube as an alternative, but if you don't have access to the service due the region or for poor internet, I'm sorry for you.
3) The Gamepad more optional
The Gamepad offers a good experience without needing a TV, still you’re also forced to use it in order to fully use the console. Nintendo already did an update to make possible to use the eShop without the Wii U Gamepad for example, but the settings doesn't support other controllers.
Since it’s impossible to buy an individual Gamepad for the moment, if yours is broken, you will need to send it to repair. But not every country has this repair service, so the alternative is buying another Wii U.
And this goes to another little problem: how will you transfer your stuff from your old console to the new one if your Gamepad is broken?
Yes, it's possible to turn off the screen still if you press a button it will turn on again. And not every title is compatible with another controller than the Gamepad (mostly eShop titles) and most of them doesn't use the Gamepad Screen for anything in special...
So why this option doesn't exist? One would save a lot of battery.
Turning off the controller in case of not using it would be great too.
5) Themes
Already available on 3DS, still I want more customization options on my Wii U, I'm just getting tired of the design. At least different colors would be nice, at least like when you're entering a folder.
I would mind buying a Zelda U or a Bayonetta 2 theme for example.
So that's all, what do you think about this list? Would you change something? Want to listen your opinions!