On January 29, 1988, the Korea Times published this article outlining the future plans for Seoul's subway system:
Interesting that the idea then was for extensions and spur lines. Line 4 was pretty much extended as planned, but the Kimpo and Chonho spurs were (roughly) eventually joined to make line 5, while line 3 was extended only a little (and extended a little further again in 2010), with the Godok and Songnam spurs basically being joined to make line 8. Spur lines as an idea haven't died, really - several light rail spur lines are planned to be built throughout Seoul over the next decade - see here and here for more information.
9 hours ago