Perhaps a group of survivors who have become so desperate to get out of the Borderland that they are willing to sacrifice anyone. They could be the antagonists as they would clash with the main characters' more moral approach to survival.
Overall, Alice in Borderland is reasonably faithful to the manga. Some minor details might differ for the sake of the adaptation, but the core story remains intact. There are no major deviations that would change the essence of the original manga.