Square peg round hole
This sounds like a recipe for confusion to me.
A new Thunderbolt device using a USB Type C connector could perhaps be converted back to an old Thunderbolt connector, but what about the backward compatibility to DisplayPort and FireWire?
A new USB device using a USB Type C connector could be converted back to a USB 2.0 or 3.x connector, but not to a Thunderbolt/DisplayPort/FireWire connector.
Would it not make more sense to include the Thunderbolt feature set in USB 3.1, but not call it Thunderbolt at all and avoid any confusion.