South Korea (hereinafter called "Korea") is planning a large future conventional submarine, known as KSS-III and/or "Jangbogo III" displacing 3,000 tons (submerged).
"The Ministry of National Defense plans to deploy 3,000-ton [surfaced, 3,500 ton submerged] attack submarines after 2020, a two-year delay from the original Defense Reform 2020 initiative, a report said Sunday. ...The development of an [indigenous] 3,000 ton KSS-III submarine is a key part of the Navy's modernization programs. The KSS-III sub is to be fitted with domestically built submarine combat systems aimed at automating target detection, tracking, threat assessment and weapon control, according to Navy officials. The 1-trillion-won heavy attack submarine will also be armed with indigenous ship-to-ground cruise missiles and be capable of underwater operations for up to 50 days with an advanced Air Independent Propulsion (AIP) system, they said. The AIP system has improved the submarine's underwater performance and gives it stealth capability." full article
On South Korea's past consideration of a 4,000 ton nuclear propelled submarine (various names including "KSSX-N") option see this globalsecurity.com report.