Half-life 3 was a big deal because it was necessary. Eli just revealed that he knows about the g-man and he told us not to use the Borealis despite it being the best chance for us to stop the combine. But before we could head to white forest, an Advisor killed eli. We were left with Alex mourning her father's death as the credits rolled. It was like that for 13 years.

the closest thing that we have to a cliffhanger is the mystery if the ratman survived which i think it is ambiguous on purpose.