PlayStation 4: Better prices for digital games
Xbox One: Better hardware
Wii U: Option to play all 3DS games.

actually forgot! instead of nothing, 3ds games playable on wii u gamepad

In a world where these changes wouldn't drastically increase the price of the product:

PS4: Backwards compatibility with PS1/2/3.
X1: A better GPU.
WiiU: A better... everything in the box :p

In a world where the final product has to be the same price as now:

PS4: Nothing (not because it's perfect, it's just the best it's going to be at $400).
X1: Hardware designed as a stand alone console. No need for hardware cuts to make bundling Kinect 2 financially viable.
WiiU: Traditional controller instead of the pad. The money saved is put into making the hardware not suck.

Wii U
PS4
XOne

to have something like the N64 expansion pack to run games more smoother etc...

Wii U - As many exclusives as PS4 and XBO has 3rd party games.

PS4 - Backwards compatibility
XB1 - $399 Kinect SKU
WiiU- Cross buy options with 3DS for virtual console and eshop purchases

Xbone - BC
PS4 - price
WiiU - power

Wii U : better online
Ps4 : I wish it had Last Guardian (more games that appeal to me.)
Xbox: To be dead.

Wii U: being on par with PS4 and see how third parties flood the console with all of their games.

PS4: Made by Nintendo.