I think that Expresso even on a 22nm node is simply to power hungry to be used in a handheld. Probably if Nintendo could get it down to 12-14nm but then again, it would be a lot better to use a arm CPU/GPU
Yeah, it's hard to tell without getting a good figure on how much power Espresso is currently sucking. I've heard guesses as low as 6w. Espresso also might have been made even smaller, but I think they needed it a certain size for the I/O pins. And remember they can clock it whatever they want to cut draw. I've been going back and forth on interpreting Iwata's comments regarding the unified architecture and how much will be based on Wii U.
Nintendo would realistically be looking at a fairly affordable SoC. It's still gonna be too early in 2016 for 3D stacking and whatnot I'm thinking, although a "2.5D" package like what's in Vita could happen.
I read an article that IBM is now licensing their PowerPC architecture. So Nintendo could theoretically license the cores, tinker with them, and stick them on an SoC manufactured by TSMC. eDRAM will be a problem, but at a smaller node, switching the L2 at least to SRAM shouldn't be an issue. I suppose there is the crazy possibility that Nintendo gets IBM to manufacture the whole SoC. They're basically the only major player (besides Intel) who offer eDRAM on modern process nodes.
Interestingly enough, AMD is saying that they're not licensing their GCN designs. They could very well have exceptions made in the case of console manufactures. But they are focusing on their semicustom line and that's what Sony and MS got, so who knows if Nintendo could even get something more custom out of them. I would not rule out VLIW5 in their next console if that's the only way they can extend their current Radeon license.