If all you wanted was to deliver on the density you could just keep most transistors dark and still stay within the power budget. That's not very useful in general but maybe there are some aplications for it. AMD has been shipping their 3D VCache setup where they layer an extra cache only die on top of a CPU die. That's been benchmarking really well and is already an effective doubling that can be cooled with a normal PC setup. Maybe there's a few other tricks like that to get a few more vertical layers out of the same processes.