Assassin's Creed 2 was supposed to cover Brotherhood's entire plot, but that was "way too ambitious to deliver properly" in just one game

Even after all these years, Assassin's Creed: Brotherhood is arguably still the peak of the series. After Assassin's Creed 2 sanded off the clunky edges of the original game, Brotherhood offered a natural continuation of Ezio's story and expanded gameplay elements tied to his increasing leadership of the titular assassins. It's no wonder the sequel seemed so natural, since Ubisoft had apparently planned for the events of Brotherhood to be part of the plot in Assassin's Creed 2. "When we designed Assassin's Creed II, Ezio's story arc was roughly from his birth to him becoming the mentor," Jean Guesdon, a series veteran who now serves as head of content for the franchise, says in an interview for Retro Gamer #287.