Davy Greenwind
Just some guy
Adventures in Rokugan 5e has a Shinobi class. Focuses on stealth and gadgets. It's pretty neat
My initial thought too was make it a subclass, but honestly having it as a background is probably the better way to go. Then depending on the ninja's specialty you can use various classes - Rogue (infiltration and assassination), Fighter (highwaymen, ambushers), Ranger (sniper assassin), Sorcerer (mystical ninja abilities), Monk (unarmed assassin) and so on. Could do an entire Ninja campaign that way and still have class variety.I'm going to be boring and say Ninja should be a background, with proficiency in Acrobatics, Stealth, the clan secret language/hand signals, and either Poisoners kit or Disguise kit. Maybe each clan could offer a different feat? Alert, Lucky and Skilled all fit, and Magic initiate goes a long way with minor illusion.
Between various monk and rogue subclasses I think the mechanical fantasy is actually pretty well covered. The assassin covered the murder ninja and the thief's fast hands covers the smokebomb tossing ninja that climbs around.
The background also allows for wizard ninjas, fighter ninjas, etc without making a subclass for each class.
Can you make at least 4 subclasses for Ninja? If not, I'd say it should be a subclass and Shadow Monk covers this niche very well.The Shadow Monk covers 90%+ of the pop-culture ninja themes quite well. Stealthy, shadow teleporting, surprise attacks, nerve cluster poke disables, martial weapon use (monk dedicated weapon), wall-running, etc. The only thing it doesn't really provide is the ability to do disguises or poison, but most pop-culture ninjas prefer to wear ski masks and black gis instead of looking like "just another peasant in the field when the samurai walk past."
You could always just forbid ninjas in your game.There are two major barriers to creating a new official class in 5e D&D. One is that to run smoothly the game basically requires that a DM have a rough understanding of the basic abilities of each class, which means that while occasional new classes might get to come to the table, there is a strong bias against adding a whole bunch. The other barrier is that it has to come with at least 3 (by 2024 rules) subclasses with at least the sense that there is juice for a bunch more.
So I'm sorry ninja megafans, I don't want a Ninja class. I'm sure the base class could be cool, but I just don't think the subclasses would be well executed (they would of course initially be the Dragon Ninja, the Shadow Ninja, and the underwhelming nonmagical Ninja, followed by a Feywild Ninja, the vaguely clericy Ninja, and the Psionic Ninja). I'm not interested in playing any of them, so it's just more crap I have to learn to run the game.
It could make a cool subclass though.
My preference would be to see it as a full class, but WotC is allergic to creating new classes, and only does so when the concept is sufficiently different from other classes. See the Artificer and the attempted Psions. So there's no way they do it as anything other than a subclass.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.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.