This has arguably already been appearing on the graphics side with stacked memory, but now Intel is getting serious about this approach: https://www.theverge.com/2019/1/7/18173001/intel-lakefield-f...

A big problem is heat. But stacked designs produce less heat overall, just in a smaller area. That can actually make heat dissipation more centralized and more efficient (due to higher thermal gradient) so I think it's a solvable problem.