Steambox If this ever happens, it will be a spec, not actual hardware.
It would be way too limiting to create a console type product that ages quickly.
It is far easier to create a steamBox rating that can be listed on PCs and in game requirements that simplifies the requirements for the average and below average consumer.
It would give manufacturers something to sell. Not just a gaming PC, but a PC with a steambox 2.1 rating.
Just like how microsoft requiring hardware upgrades for new pcs in order to support windows features, a steam rating would push manufacturers to offer better hardware that can be more easily advertised to the consumer.
As for input devices, it will be cool to see what they come out with, but I am not holding my breath. The only thing a keyboard and mouse are missing is rumble features. Otherwise they work pretty well. Other companies are making things like the novint falcon, VR glasses, and the xbox 360 controller. All valve has to do is push games to support this stuff. They could easily slap a tag on the purchase page for games in steam listing the 3rd party hardware that is supported. Right now they don't list much beyond the xbox controller.