I'm sure most of you guys have heard of the report done about possible human-alien contact, and what it would mean for humans.
This chart from National Geographic is basically a campaign matrix.
Pick and choose. Want post-apocalypse from alien contact? Choose a "Harmful" alien interaction, such as "Unintentional Harm: Acts of Incompetence." What if Earth went all Gamma World because aliens unintentionally caught Earth in a solar-system wide science experiment?
Want friendly aliens? Well, how did meeting them go? Was it good, neutral, or bad? If it was good, was there exchange of science or were they essentially uncaring?
How about "Intentionally Hiding." Come on, how can you not create a fun adventure when the PCs stumble upon proof of the aliens who don't want us to see them? Maybe they fear our violent ways, or unleashing theirs on us, or fear our diseases or social contamination? Are they like Tokugawa Japan, an advanced culture seeking isolation? Or are they like fearful sasquatches of legend, running off when they see man?
This matrix is pure gaming gold.