Ki Sung Yong is very unique. His passing and play making skills are very rare in Korea. If you don't go too deep in the details, Han Chan Hee (has struggled for the U20 though), Hwang In Beom (is rather a classic playmaker than a deep lying CM) and Kim Jung Min (still too young to judge) come to my mind, but we'll see if any of them will be able to replace Ki one day. I don't think that Lee Chang Min is a KSY type of player though. He has developed in the last years and for Jeju, he's more of a box-to-box midfielder who likes to shoot from long distance. I think that maybe he could complement KSY better than KJC does, because LCM can defend better, has a good workrate (tbf KJC has it too) and is more direct (which is KJC's biggest problem with the current KNT style). But as I've said before, I think he's too offensive to be Ki's partner CM in the 4-4-2 (just as KJC). Maybe if we play 3-5-2 again, he could be an option.