According to a recent announcement by Game Informer and a few rumors, a huge announcement will occur during the upcoming Gamescom expo for Valve.
The announcement might be the official debut of Half-Life 3 after Game Informer revealed they are not allowed to show a preview of their upcoming issue's cover since it would spoil the "world premiere" announcement. Also contributing to the rumors of the announcement being Half Life 3 is due to Game Trailers who stated they have a special trailer video dedicated to just Valve which will occur on September 17.
The announcement is expected to occur during the EA press conference who are the current publishers for Valve games on consoles.
Previous rumors teased Grand Theft Auto V would have been the huge announcement after E3 had passed. The rumors were debunked by Gamescom organizers who stated a mistake lead to the debut trailer being listed on their website while Rockstar themselves have never attended Gamescom nor made an application to attend this year. Here is the response abotu the International cover: