Portal 3 may actually be a real possibility now following a new development. As you likely know, Valve pretty much avoids the number three like the plague. It's a big gag following the fact Half-Life 3 never came to be despite many reported attempts and while it seems like it could happen now after the success and ending of Half-Life: Alyx, we have no idea if it will ever actually happen. This trend continued with Portal 2, Team Fortress 2, and Left 4 Dead 2 never receiving proper sequels, creating a lot of questions about why Valve is just refusing to make threequels. There are a variety of reasons for this, largely stemming from the fact that these are huge games and follow-ups are probably quite difficult to make. Valve is also structured so that anyone can work on whatever they want, meaning developers have to rally a group together to make a game and ensure they all stay committed to it for years.
During an interview with Did You Know Gaming (via GamingBible), Portal 2 Erik Wolpaw revealed that both he and co-writer Jay Pinkerton have a concrete idea for the story of Portal 3. There's no script and it seems that it's more of a premise rather than a fleshed out idea of the entire story, but they know where the game starts and what it would look like. The two have also pitched it to Valve and received an enthusiastic response, but they still need a team to actually bring the game to life. Given the structure of Valve, that's very hard to do.
"Yeah Jay and I have an idea that we think is pretty awesome for what would happen, generally speaking," said Wolpaw. "We don't have a script or any details worked out, but we have sort of a starting point that we like a ton. So you know, it's good we have this idea but there's a lot left to do. Someone's gotta think up some new portal puzzles! But we do have an actual idea, yes."
Wolpaw also noted that it would really only be possible during a time where Valve employees aren't busy on another project so they didn't have to take manpower away from another game. Whether it ever happens remains to be seen. It sounds like there's enthusiasm for the game within Valve, but making it actually happen would be quite a tall task. GLaDOS actress Ellen McLain has also expressed interest in Portal 3, so there are a lot of people that want to make it.
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