I agree that a fantasy assassin may be enough of an archetype and story to be its own class, but I also feel that a lot of the that class would overlap with the Rogue. The current implementation as a Rogue sub-class is lacking though. Like many other archetypes in 5e, and others have pointed out, it is possible to do a very credible assassin could be done in a variety of classes.
So, I guess my vote would go towards re-doing the current subclass to be more effective.
So, I guess my vote would go towards re-doing the current subclass to be more effective.