I took a look at what you could cram into a 1x1 mm cube using fabrication technology that is on the leading edge.
You can build chips that are 1x1 mm and 25 microns thick, including the active layer.
You could stack them and interconnect them using through silicon vias into a stack of 40.
The leading edge of scaling is 10 nanometers (more or less - but easier for the math) which means you could have 400 billion transistors or 50 gigabytes of memory in that one millimeter cube. This is not out of the realm of being manufacturable in 10 years.
One basic problem in things talking to each other wirelessly is the physics of antennas. That's a much more difficult problem, but then I think everything that is analog is a more difficult problem.
And I'm not sure how you would get them to "bond" to each other.