The current info is that Broadwell will be a process shrink of Haswell, with little to zero architectural changes taking place except the transition to 15 nanometre. The cpu's are expectected to work in socket 1150 with a bios update, just like Ivy bridge cpu's did with sandy bridge motherboards.
DDR 4 is still so much faster than needed by current processors as to be almost irrelevant,but you can be sure hybrid boards will pop up with either or even both slots on,when it approaches mainstream(this is unlikely soon for obvious £$£$ reasons). usb 4 hasnt even been mentioned yet, and the chipset on a motherboard isnt 'all those little parts' on the motherboard, its the whacking great big lumps of silicon with heat sinks on, and the ram controller in the processor.
If you are wanting a five year solution, a non K haswel rig (4440?) will cover your needs well, or you could buy an abacus;
as I am sure others of a less stable nature may suggest.