I personally think the micro usb should go away. I had many problems with the cable getting broke, loose connections, and even one axoloti board where the connector went completetly fucked and bent off.
The host should be either A or C, my first instinct was A since that is the format I have around with all my midi/usb devices, but as mentioned here it seems C will be the main standart in the future and it is more in the spirit of a mini device. What I am wondering about this is though: All these USB hubs with a C connector are USB3.0. It seems out of this discussion that this is not supported, right?
it looks good, very mini! May I ask why there is four mini jacks?
Considering the shield buisiness: I just stumbled on this from the teensy world, good example in how an axoloti shield could look like, right?