Did you think about an optional second 5V access? Because USB is limited it could be a possibility to attach an additional psu.
I did think about it, but at this time it will most likely not be necessary - most people would either use a powered USB hub (and those now easily deliver 1A and more per port) or use USB3 ports.
In one of my tests I used a powered USB2 hub where the power brick provides 3A (plus 0.5A from the PC) and that was sufficient to attach and run 3 devices, and actually push them all the way to their limits. So by just not using all ports you leave the extra power for the ports that are in use thus achieving the same effect as if you use 2 ports simultaneously.
Plus, the extra USB connector would get in the way the simplicity idea
