It could be about Cave Johnson somehow being involved in the Apollo 18 mission. Maybe he's a secret backer providing unique technology. For example, his company's experimental propulsion system could be used on the Apollo 18 spacecraft.
One possible plot could be that Cave Johnson, in his never - ending pursuit of scientific discovery, gets wind of some strange readings related to Apollo 18. He then uses his influence and resources to get involved in the mission. Along the way, there could be conflicts with the official space program's bureaucracy, and his maverick approach could lead to both amazing discoveries and near - disasters on the moon.
Perhaps the fanfic could revolve around Cave Johnson sending his own team on a parallel mission to Apollo 18. His team could be exploring different aspects of the moon, like hidden caves or alien artifacts, while the official Apollo 18 crew is doing their own thing. There could be a race between the two groups to discover something valuable or dangerous on the moon.