Are there any improvements that could be made if you dropped support for ATX power supplies? I notice your new mATX/ATX design is SFX/SFX-L only (unless I missed something).
The easiest thing would just be no longer including the ATX bracket. But I think you would still want to have a solution...
Rounded edges are subtle but a nice touch. I prefer the plain front panel.
If you want a little more visual interest I thought the burgundy coloured front panel you had in one render looked good. But black is probably more universal.
There are Thunderbolt drivers on the download page for the Gigabyte Z170N Gaming 5 now.
I've installed them and I get a thunderbolt tray icon. But I have no Thunderbolt 3 (USB-C) devices to test with.