Adding additional smoothness to existing voxel engines I'm not sure would have much effect, unless you have something specific like moving a ball on a smooth surface (but the SDF I'm seeing here wouldn't support that either).
As for "create a hole, then close it behind you", this is about as game-changing as open/closing doors (or tunnels with doors). I'm open to suggestion, but honestly this is amazing tech, I just don't think it will create very fun games.
Kind of feel the same about the demos I see of spherical or non-euclidean geometry games. Its very interesting, and impressive, but seems like it is an engine in search of a game.
Have seen this with a lot of software "frameworks" (web, game, graphics, etc.). Nothing wrong with writing an amazing tech demo just for the hell of it, but then when it comes time to do "real world" tasks, the frameworks are often in search of a fitting problem.