Narita has expanded as far as it can go. Haneda, already the busiest airport in Asia, can’t go anywhere but further into the bay. Tokyo will eventually need a third airport, or so we’re told.
The Tokyo Metropolitan Government, with Ishihara’s personal blessing, wants to use Yokota Air Base, a U.S. Air Force base in west Tokyo, for civilian flights. That seems somewhat unlikely now, since the base is going to have expanded duty as the Air Self-Defense Force’s command center (protests be damned), on top of its existing role as headquarters for U.S. forces in Japan.
The other option is to build offshore. Several sites have already been proposed, most of them in Tokyo Bay, except for one site off of the Kujukuri Beach on the Pacific coast of Chiba (obviously a bad idea; spoil a nice beach with an airport that’s even farther from Tokyo than Narita?!).
Is the third airport really necessary? After all, Narita just had to cut its landing fees to stay competitive. I think it all has to do with the fact that the Kansai region will have three airports as of next February. Ishihara just doesn’t want his half of Japan to fall behind.
London has five airports and is getting along just fine. That doesn’t mean that more is necessarily better. Berlin is in transition from three airports to just one. Maybe Tokyo will someday admit that Narita was a dumb idea, and run all of its flights from a humongous future version of Haneda, acres and acres of concrete sprawling out into the bay.