During the "Evolving Game Design" panel, "Fallout 3" lead designer Emil Pagliarulo elaborated on the ending that could've been. [Spoiler alert!]
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When asked if there were things in "Fallout 3" that he loved but had to cut, Pagliarulo talked about the game's ending, which had the player following around a giant robot named Liberty Prime as it battled and blew up enemies. When I got to this part, I wondered why I couldn't control the giant robot. Pagliarulo explained that the original idea for Liberty Prime was that it was five times bigger than what it was and players would get to ride in his head.
"[Executive producer Todd Howard] and I had championed this idea for a long time," Pagliarulo said. They also had the idea that Rivet City (a dilapidated aircraft carrier) would become operational and it would drive down the Potomac River. However, due to time and technical limitations, P agliarulo said they had to scale back their plan, and that that was just of many things that had to be cut.
Perhaps we can hope for controlling a giant robot in future DLC.