If the Japanese have Traction Trebuchets, then they would be Historically Correct, which would be nice.
In fact, I’m already pretty glad that the Japanese civ don’t have crossbows, since they never developed them, and that they found a unit replacement for anti-armor damage.
Regarding the cannon, in my model (Japan Civilization Concept for AoEIV_ by GoldenArmorX) I commented that the japanese could have several cannon models , mainly because of the way in which they obtained them: Buying them from the Portuguese and Dutch, or producing their own smaller ones.
Probably the Cannon is a Dutch or Spanish cannon… or as some said, it is pure Eyecandy and they went too far when making the trailer.
At least in my model you could buy “cannons” from Spain and bombards from Holland at the Nanban Market, an unique building to represent the purchase of foreign technology at the Sengoku Era. Who knows if they would incorporate a similar idea as a mechanic in Imperial Age, otherwise it got out of hand.
The very fact that the Byzantines have the ability to purchase mercenaries gives rise to many curious new units unique to the original civilizations. And we still don’t have the technology tree of any of the new Civs.