I've often thought that Jeju would need at least one intercontinental flight from both Europe and the US. Let's say LHR-CJU and SFO-CJU once or twice a week. With Nine Bridges Golf, Halla-san, the excellent seafood and now Daum relocated there they might have enough pulling power to justify this.

And with the existing direct flights to Japan and China it might attract stop-over passengers too.