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.
One great 'borderland fanfic' is 'The Survivors of Borderland'. It focuses on a group of players who form an unlikely alliance to overcome the challenges. Another interesting one could be 'Borderland: New Horizons' which explores a new area in the Borderland universe with unique rules and characters.
Another plot could involve new characters being introduced into the Borderland. These new characters could have unique abilities or knowledge that either helps or hinders the main characters in their struggle to survive. Maybe they know a secret about the origin of the Borderland that could change everything.
To start writing a 'borderland fanfic', begin by choosing a theme. It could be about survival, friendship, or the search for power. Next, create your characters. Give them unique personalities and backstories. For example, you could have a character who was once a corporate spy but now is lost in the Borderlands. Then, outline the plot. Decide on the main conflict and how your characters will resolve it. Remember to keep the Borderlands feel by using its unique language, settings, and technology.
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.