Intel has pulled back the curtain on a new mobo format called 5x5. 5x5 boards are smaller than Mini-ITX mobos, but they retain support for socketed CPUs. The official reveal was made at a presentation during IDF15.
The new format measures 5.5" by 5.8" (140 mm x 147 mm), roughly 29% smaller than Mini-ITX. The LGA socket can accept CPUs with up to 65W TDPs, from Celerons to Core i7s. The socket's location, board dimensions, and mounting holes are all standardized. The tiny size belies the 5x5's expandability. You get two SODIMM slots, 2.5" SATA or M.2 storage, wired and wireless networking options, and a "rich I/O selection." There are no PCIe slots, however.
Intel claims the 5x5 format will allow for sub-1-liter computers, though still with socketed CPUs. The company says a system packing a 35W CPU will be only 39 mm tall while occupying a volume of 0.85 liters. Systems get a little bigger with a 65W CPU and a 2.5" drive installed, but should still remain in the realm of "tiny", all things considered. The company also shows two examples for cooling arrangements, for industrial and home use respectively. The latter appears to use two lower-RPM fans, with the air exhaust pointing away from the user. As a bonus, 5x5 systems can run from a DC power brick.