Li Shenron
Legend
Back in 3e, Complete Adventurer had a 20-level Ninja class. If WoTC brought the Ninja class into 5e (for a possible 5e Kara-Tur setting?), would it be its' own class or would it be a subclass of another class (such as the Monk or the Rogue)? And if it was its' own class, what would everyone like to see in it? Ditto for subclasses.
Even though the name is different, the Ninja has been in 5e since the start as the Monk subclass.
It would make sense to make a full Ninja class for a fantasy setting where the character concept is more prominent, and widespread enough for it to require further significant differentiation through its own subclasses. For example, in an oriental setting there could be both a Ninja and a Samurai base class (perhaps at such point even replacing the Fighter).
There is another aspect to consider which is the sort of "prestige vibe" that the concept of Ninja carries, meaning that in a lot of stories those who enter the path of Ninja need to be already somewhat advanced, which strides a bit with having 1st-level Ninjas. But this may vary depending on the fantasy setting own story, and in another setting being trained as ninjas since childhood might be the norm.
So I don't think one option is generally better than the other.
As a background though, I don't think it is appropriate. It doesn't fit with the original meaning of a background (what you do or did for a living), but admittedly this meaning is lost already since WotC messed up with lots of background variations. Still, a background is too small to capture the many facets of a Ninja.






