The only reason the assassin mechanics got so tiring is because they were rehashed annually. The one great thing the series did was to introduce the sailing mechanic - I quite liked sailing, naval warfare, discovering islands and boarding ships.
The assassin mechanics can still be fresh if they tweak them just enough to be fresh. For example, if the game were to be set in Feudal Japan I think it would make more sense for the character to be a Samurai or a Ninja. So they could be a ninja assassin I guess. But we will never get an AC set in feudal Japan because there weren't any buildings tall enough to use as viewpoints in the game 








