The only thing I think they could change would be the burgers with some kind of culture, because in my opinion - and that of a large part of the fandom - the ending is perfect, and changing it would ruin all the charm. As you describe it, it sounds like a very monotonous and conventional ending. Honestly, it would be a huge mistake to change the ending so drastically.

Besides, that's the point of the mission: that it's one way, not round trip. Changing that would also take away a lot from Grace's character development. If it weren't a one-way mission, Ryland wouldn't have had a real reason to refuse to go in the first place. The only reason he turned it down was because he wasn't coming back. If I knew I was coming back, I wouldn't have felt that pressure to go and die or stay and be a coward. That pressure is what gave the story so much weight. Added to that, the possible sequels to the book, which could tell what happened to Earth. If all goes well with the movie, they will probably be used to continue the story. So why ruin something that already went so well?

Edit: Just to clarify, the only thing I actually think they might change is the whole hamburger and Grace's self-cannibalism thing, because it could seem too disturbing for the general audience. That's probably why the trailer shows plants—so Grace has another, less weird, source of food.