It seems that the Half Life community knows a lot more about Half Life 3’s plot than the game’s release date. This cannot be helped as the fans have waited more than a decade for Valve to release Half Life 3 and throughout the period, they have explored every possible plotline for the game.
Many fans agreed that the storyline might differ in context but the elements will remain the same. In other words, Half Life 3 will surely start with the Combine acquiring the Borealis and work their way to understand the weapon of mass destruction.
The protagonist, on the other hand, will act as the resistance to stop the Combine and destroy the Borealis once and for all. By the end of Half Life 3, the protagonist will achieve his goal but with the aid of returning characters like the G-Man and Alyx. The destruction of the Borealis will see the Combine submitting to full retreat, thus, restoring the peace in the universe.
The above plot was based on how Half Life 2 ended, with references to the Half Life film. Now, all that is left is for Valve to start developing Half Life 3.