Considering that he still have to be king and to conquer Spain and Italy, we can imagine he won't die yet. but to him, to kill your own mother there's only one way: Lagertha dies like Astrid did, realizing that she's the wall between Ragnar's sons and his dreams, she won't get mind to die. that said, I can't imagine another dead like that.
Probably she will die by Ivar's hands as we known, but at the end of the present season, IF that shit happen.
To me would be AWESOME if Magnus took the job. I mean, Ragnar said Magnus doesn't his son, to protect him and his own plans. If she dies in his hands would be pretty philosophic. But Lagertha won't travel to England again I think, and we don't even know if Magnus is alive, he is completely forgotten.