It doesn't even have to be the .dats, I'm just saying an ASSASSIN CLASS would be an ideal fit for Ninja because that's typically what Ninjas were - Spies & Assassins. Plus have you ever played the FFTA games? The Assassin job from those games were very closely related to Ninja in terms of skills & abilities, and were generally just really awesome (And vierra exclusive o.o)
We still don't know the full extent to how much things are going to change in ARR, but so far it looks like the basics of the armoury system are staying in place, and while MRD has a pretty good set of skills that DRK could work off of, there isn't anything like that for Ninja. PUG has some skills that would be great like Featherfoot, but it doesn't have the core skills a Ninja would need or the weapons. I agree with you that not every job requires a new class, I actually posted a fairly detailed example of how RDM could easily work off of GLA and play the way people have always wanted it to. But that's not going to fly for Ninja.
Assassin could easily have dual-wield as a trait (thus preventing other class or jobs from using it) along with some of the base skills & magic, with Ninja adding in shuriken and additional skills specialized for party play.