A USB Image gives the highest mix of security/convenience/ease-of-install. This could easily be our flagship install method.
This would probably be a linux host with KVM images installed and ran at boot time.
The problem is finding a host distro with great driver support which is user friendly enough to be configured if necessary.
Remember that this is not an installer image, but for a usb stick with whonix installed and ready to boot. This is uncommon in the linux distro area.
Related, "Host Additions":