Microsoft Asking Developers About An Increase In RAM And GPU Clock Speeds For The Xbox One
Looking at the size of the XB1 chassis, and the size of the motherboard, expansion seems very possible. Microsoft made the XB1 quite large, with copulous amounts of ventilation for heat dispension/dispersion, allowing room to upgrade the hardware. However, if this turns out to be true, it's very possible we may never see an XB1 slim, because the slim version will have nowhere near enough ventilation to accommodate more demanding hardware. It's still possible for us to see an XB1 Slim, as I'm sure they could refine a few things in the design process for the slim version.
One thing to note is that apparently the XB1 engineers overcompensated for heat in their design, and have found that the XB1 runs cooler than expected under load. Upon realising this, they've given developers a choice to choose between having more RAM, or a faster GPU clock speed, as they can't accommodate for both. I really hope we get both though.