I kinda like Varian and kinda dislike Anduin, so hopefully we'll be through many expansions before the old ponytail kicks the bucket.
Now you see it. Now you don't.
He doesn't have to die for Anduin to take the throne. There are plenty of reasons for it to be plausible for him to let his son rule.
Idgaf how Varian dies, I never liked his story or how he was introduced. The comics we're great but still felt very, I don't know, "me'dan".
There must always be... a Lich King
I imagine that Varian will have a more heroic death when it comes time for him to die. He'll end up sacrificing himself to save someone else (maybe Anduin or Jaina).