Succession is usually not straightforward. Ki Jong-il and Kim Jong-un, despite both being the fairly clear favourite of their fathers when those fathers died, both had to spend time consolidating their power and it was not immediately clear that they would take over (Kim Jong-un did so relatively quickly, but it took Kim Jong-il several years to get to the point where he was obviously in charge).
If and when Kim Jong-un dies, whoever succeeds him will almost certainly have to do the same thing, and there's no guarantee they will succeed. The first sign that Kim Jong-un was likely to take over from his father was when he was named chairman of the funeral committee, so in the event of his own death, that might be the first clue we get about who is likely to succeed him.
