It depends on the "vision" they have for the game. A lot of the AC games essentially have the game's free-running and combat mechanics already created and built, essentially what every team does is just put a new coat of paint on that. A lot of the "rag doll" physics are already made by other companies like the "havok" engine which is incorporated by A LOT of games. In fact, there are companies that make money off of selling their "engines" to game developers. The 'Arkham' Batman series is built off of the Epic Games 'Unreal' Engine. UbiSoft has its own engine for the AC games called "Anvil".
Coding takes a lot of work. I'm not a coder but I work with some and know from some experience that some coding is easier than others and every time you toss in a new element, it could cause you to look back at the others. Considering how detailed games are nowadays, it takes A LOT of work. This is why sports games (like NFL, NHL, FIFA, etc.) and Call of Duty are issued out annually, they're just putting a shiny layer of paint on an existing model with some tweaks along the way. The same can be said for the AC series in hindsight, whose "core" mechanics have been largely the same since the series' inception. Updating Anvil could take a lot of work in order for UbiSoft to achieve their "vision" for a Feudal Japan AC.
I could foresee the Japanese having A LOT of interior scenes, and as such, you're going to want to "feel" like a true ninja. Not just hiding behind corners, but perhaps even hanging from the ceiling or even reworking how your enemies react and their AI. It's not impossible, but it takes a lot of work that perhaps UbiSoft isn't "ready" to devote to the series right now. If they deliver a craptastic Feudal Japan AC with poor play mechanics, they'll get a lot of hate. Have you played 'TENCHU'? That is perhaps the BEST ninja series that has ever been released and it incorporated a lot of what myself and other fans might anticipate out of seeing in a Feudal Japan AC. I honestly feel AC's current character dynamics might be a little too 'rugged' for feeling like a stealthy, sneaky ninja. You're going to want to speed the character up.