Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum (remember EmDrive?). I wonder if it could be used at very small scales with lower energies for some applications.

Another interesting reactionless drive within possibility:

> "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field.

https://en.wikipedia.org/wiki/Reactionless_drive#Movement_wi...