Of course they are going to introduce new characters but Ellie must be in it since she as the only immune person is still pretty much the most important person in this world.
Having another character with immunity appear would be cheap.
I completely agree with you. Why do so many people think that Naughty Dog can't make a good sequel? They have done it before. Jak and Daxter -made by Naughty Dog- had great sequels(Jak 2 and Jak3). I hear that Uncharted's sequels are great- I haven't played the series yet, but I plan to. I think Naughty Dog can pull off a great sequel to The Last of Us. I wouldn't be too disappointed if they didn't end up making one though.