Jon, Dany and Tyrion will reach the end of the series IMO. Not to say all of them will survive, but if they die, it will be at the very end, like (Spoiler for a decades old book and anyway already aired trilogy of movies) Thorin in the Hobbit.
I don't see Cersei biting the bullet this season either, even though maybe it could be a shocking finale for the season.
I wonder how far GRRM would go into killing of characters. In the grand scheme of things, we can say that now that we know who the real protagonists or even survivors are, it wasn't that absurd and done just for shock value to have the early protagonists killed. Can you imagine Dany dying in a very stupid way just before she makes her final move on King's Landing? the equivalent of having a sports movie about a team that loses out of nowhere the semi-finals. Not even the final match, which could be an interesting choice for a movie, just in the semi-finals, like after all happens all the time... there are are so many people that stumble not on the last step, but sometimes one or two steps before.