Valve CEO Gabe Newell has cast doubt over the possibility that there will ever be sequels to some of the company's most beloved franchises.

Speaking on a one-off podcast with Geoff Keighley, Newell said Valve has changed the way it makes games over the last 10 to 15 years. He said he understood the desires of fans who want to revisit worlds they love, but that Valve would be unlikely to do so unless there was a technological imperative.

Referring to Half-Life, Left 4 Dead, and Team Fortress, Newell said, "You know, we love all those games, we love all those characters and universes and story lines and we have no shortage of opportunities."

"If you think of it as each one of our franchises represents a tool ... you just want to pick up the right tool at the right time," he said. "So like Dota 2 is an incredibly character-rich [game]. There are how many? 110 characters? So if you have a problem that involves wanting to work on the aspect of having lots and lots of strongly realized characteristics, then Dota 2 is the right place to do it.

"Team Fortress represents a tool. When we were thinking of what are the next challenges, we tend to pick the franchises that are sort of most useful going forward. And if we don't have one, then we have to create a new one."

"I get it. I'm a fan of TV shows, I'm a fan of writers, I'm a fan of movies, I'm a fan of games," he said. "I certainly understand why people are like 'Hey, I remember this awesome experience and I'm starting to get worried that I'm never going to get to have it again.'"

"So we understand it and we feel that. We think at the end of the day customers are going to be really happy with where we spent our time and how we've turned that into entertainment for them. But we're also going to build on what we've learned ... and we've learned a lot. We're not going to go all retro because there are too many interesting things that have been learned."

Referring to Half-Life 3 in particular, Newell said Valve would only ever consider it if employees at Valve wanted to do it, and even then, they'd need a compelling reason to do so.

"The only reason we'd go back and do like a super classic kind of product is if a whole bunch of people just internally at Valve said they wanted to do it and had a reasonable explanation for why," he said.

"But you know if you want to do another Half-Life game and you want to ignore everything we've learned in shipping Portal 2 and in shipping all the updates on the multiplayer side, that seems like a bad choice. So we'll keep moving forward. But that doesn't necessarily always mean what people are worried that it might mean."