SEOUL — IF China and Russia ever emerge as economic giants, which city would be ideally located to serve as an airline transit hub for Asia?Eager South Korea already has plans for a mega-airport on a shallow bay about 35 miles west of Seoul. The final go-ahead for construction will be given in 1992, but the scheme is on the drawing boards with a price tag of $5 billion for the first two runways by 1997, and two more runways possible. If fully built by 2020, it would handle 100 million passengers a year, with an estimated one-third of them just in transit. Seoul's present airport, Kimpo, handled about 25 million passengers last year. Although a second runway was added in 1988 for the Olympics, Kimpo will operate above its capacity by 1995. "There's a great chance that Seoul will become the airport center of Asia," says Shim Hyieg Yun, director of engineering for the project.