"PicoPSU's" are still in their infancy, as well as cases that support them. In theory you could save a huge amount of space in a modern ITX build by only using internal DC/DC power conversion and having the AC/DC adapter external(Like a laptop charger or as used in some game consoles). Brings up it's own issues, and some people have a hatred for those power blocks, but with a 12V input all you really need is two buck converters(Or three if input isn't 12V) to get the 5V and 3.3V lines besides the protective circuitry so in theory you could have the whole power supply essentially within the internal cabling.