Article: In 2003, Nintendo announced that a new The Legend of Zelda game was in the works for the GameCube by the same team that had created the cel-shaded The Wind Waker. At the following year's Game Developers Conference, director Eiji Aonuma unintentionally revealed that the game's sequel was in development under the working title The Wind Waker 2; it was set to use a similar graphical style to that of its predecessor. Nintendo of America told Aonuma that North American sales of The Wind Waker were sluggish because its cartoon appearance created the impression that the game was designed for a young audience. Concerned that the sequel would have the same problem, Aonuma expressed to producer Shigeru Miyamoto that he wanted to create a realistic Zelda game that would appeal to the North American market. Miyamoto, hesitant about solely changing the game's presentation, suggested the team's focus should instead be on coming up with gameplay innovations. He advised that Aonuma should start by doing what could not be done in Ocarina of Time, particularly horseback combat.[l]

Now answer this question: What was one feature that could not be implemented in Ocarina of Time?
horseback combat