There is a reason for not having modular main boards
Power = Voltage x Frequency x Capacitance
Chips have capacitance. Tracks on a PCB have a little capacitance. Chip sockets have some capacitance. Connectors have capacitance for each half of the connection. If you can add more and more DIMM sockets, you have some nasty choices:
1) Put some huge power transistors on your CPU (and memory chips) to drive the enormous capacitance of a row of daisy chained DIMM holders - even when few DIMM holders are present.
2) Divide the frequency by the number of DIMM holders daisychained together.
3) Put bus drivers on each DIMM holder (expensive, power hungry and increases the latency)
In short, a modular motherboard is not competitive. A better way to go would be to reduce the number of connectors and sockets. This can be done by putting the DRAM on the same chip has the CPU. Much of the complexity of a CPU is there to deal with the latency of the chip package+chip socket+PCB+another chip package+northbridge+back off the chip package+PCB again+DIMM socket+DIMM PCB+memory chip package. Scrapping all that latency gives you a healthy chunk of space on your CPU for DRAM.
Now your CPU only needs connections for power, cooling and communication.
DC power may not be the best answer. If there was a standard voltage and if you do not mind asphyxiating a few techies when the liquid nitrogen boils, I would say low voltage DC through super conductors. The other choice is high frequency AC. Humans are far more resistant to high voltages at about 1kHz than at 50/60Hz, so you could use thin (expensive) copper cables covered in thick (cheap) PVC carrying a few hundred volts at high frequency without risk of electrocuting people. Converting the voltage only requires a transformer (smaller than you are used to because of the high frequency). Converting to DC still requires a filter, but the capacitors would be smaller than you are used to because of the high frequency.
A nice dream. Google have the income to make it happen, but I do not see them leaping into low margin standards based hardware development at present.