Microsoft seems to be working on a smaller, cooler and cheaper CPU for Xbox One, a product that might be used for an Xbox One Slim at some point.
Proof of this is Microsoft hiring console engineers and a LinkedIn post from Daniel McConnell, senior manager of system on chip physical design at AMD stating:
"...successfully planned and executed the first APU for Microsoft's Xbox One Game Console in 28 nm technology and a cost-reduced derivative in 20 nm technology."
The consoles manufacturers are of course always looking for ways to trim down the cost of making consoles, and one way is to make them more energy efficient, and as we saw during the latest generation, Microsoft isn't exactly shy when it comes to re-designing their consoles. A slim version of the chunky Xbox One might well be something they have lined up for E3 next year, if the rumour is true.