I think if Isayama was going for a GoT ending Eren would die by Mikasa's hands, Mikasa would get exiled, and Armin would become the new king. Gabi would probably ride off somewhere or whatever the hell Arya did at the end of the series.
Armin's VA being the narrator is an anime invention, and though Isayama has had some input into the anime, I don't know whether that was the case in the first season (before it was a hit). It's possible that Armin surviving has nothing to do with what he plans to do. Or, he might have liked that idea and decided to run with it.
I'd be fine if all the characters die as long as the storytelling is good, but I don't think he's going to kill everyone. Seeing how long it took him to kill Sasha (who was supposed to die many many chapters before she actually did) I think most of our core cast is going to make it.