Soundwave on 19 June 2015
I think it's partly because Wii U is so weak that they can't unveil it just yet.
If they did what Iwata did at E3 2005 where he showed the Revolution base unit, it would take too much away from the Wii U which is already fragile enough.
I'm thinking they're fairly far along though to the point where game development is definitely underway for it.
