Pretty much all major carriers in the U.S support VoLTE (except Sprint) and all major carriers support WiFi Calling. Source:
https://support.apple.com/en-us/HT204039. If ASUS can get in touch with the U.S carriers over support, licensing (or whatever it is preventing this), that would be cool. Should just be a software issue since the hardware's already there.
Two more things came to me today of what else might be good features to include.
1) An option where plugging/unplugging the phone doesn't turn the screen on.
2) Status bar customization. Like moving the clock from the left to the center, adding the day/date, and seconds to the clock (i.e Wed 10/16 6:15:01 PM). Or hiding certain icons like the vibrate icon.