Transformers on DC are already a solved problem with various switched-mode converters that can boost or decrease voltages at will. In fact, many AC-DC supplies (phone charger, etc.) are already using such converters, first rectifying the incoming AC. Not needing a bulky transformer makes them smaller and lighter.

Most stuff is switched mode nowadays anyway, so this becoming less and less relevant. Not sure if there really are any transformer based appliances in my house anymore.