Well that was a pretty damn good season finale. Ivar kills Sigurd in a blind rage. I think his solidifies Ivar as an unstable madman, but a genius at the same time after his battle tactics work to dismantle the Saxon army. Ivar is turning out to be one of the biggest characters in on the show, and he replaces Rangar's late life personality which had an unpredictable nature. Its clear a major chunk of next season will focus around him and his conquest of England.
The other part of next season will focus around King Harald and Lagertha. Harald brother no doubt wants to accompany Bjorn to keep him distracted while Harald focuses on Kattegat. Looking at the previews, it seems somehow Torvi survived being a pin cushion. The death of Ragnar has causes instability. Harald being the King of Norway, I assume Rangar was the king of Sweden. Its nevey really stated, but thats where he was born. Kattegat is actually a sea, that's more of a bay between Sweden, Norway, and Denmark.
I didn't think the show could go on after all the deaths, but maybe it can for at least another two seasons. The warrior bishop is intriguing enough. I wonder what is going to become of Floki in all of this.