Kotal kahn is obviously growing attention towards himself because of his strength his looks(a bit similar to Shao kahn like demi-god) and many more thing. Some rumors were that he is a son of Shao kahn but with this some question arises are:
If he is a son of Shao kahn then where the hell has he been with many years? and the answer for this to me uptill now is, I think Shao Kahn is a race of Outworld gods and his death brought Kotal Kahn's attention.
Than Ed Boon said that Kotal Kahn is the new ruler of Outworld. Watch this video
[YOUTUBEVID]https://www.youtube.com/watch?feature=player_embedded&v=nsS0lnJ2SSI[/YOUTUBEVID]
We know what normally happens is that emperor replacing other emperor can only be possible in two ways. Either new emperor defeats the previous one and takes his place or the old emperor dies and his son becomes the new emperor and occupies his seat. So, this prove 2 things either kotal kahn is more powerful than Shao Kahn which he seems to look like and defeat him or he is Shao Kahn's son.
First thing doesn't suits with the current scenario so i will go with the second one that Kotal Kahn is Shao kahn's son and Shao kahn death brought Kotal kahn attention.