Will the GL-MiFi also be updated to ac? Or better, when will it be? 
No plan for MiFi to have AC now. Sorry. But your voice is heard. Something may happen later.
Will the Ac model have a much larger mold than the usual AR300M model or be similar in size?
larger, not much larger. It is smaller than the MiFi case we have.
I can easily live without gigbit ethernet since most things are wifi these days, and it won’t improve the broadband connection much.
I’m guessing if the device is still going to be powered by micro-usb (1a) then we are going to be limited to USB2 speeds for attached hardware instead of USB3. That’s the deal breaker for me, since I use mine to share files from a harddrive/usb stick to my phone, laptop, and tablet.
newly designed AR300MD will still be USB2.
We are making a new device which support USB3.
Both USB2 or USB3 in these embedded devices are much slower than USB specs. The USB3 speed of our new device is around 52MB/s when testing using dd.
But if you use samba to read/write the USB device, the speed will be further decreased to 30+ MB/s
So USB3 is about half the speed (52MB/sec vs ~110MB/sec) that it should be because of the embedded device.
Even so, I think I would buy a USB 3.0 and 5G 802.11AC device in a heartbeat. I’ve been searching and haven’t found anything close, so if that is on the horizon, good luck!
If I’m reading this thread correctly, there is no way to add 5 ghz to the ar300m16 via the PCIe and never will be?
yes we will not add support 5G support to AR300M16.
What if you developed a USB device that could plug in and be used as a tethered device. This way cooling could be engineered into the separate device.