First off, when asked if Valve was working on "any fully-fledged single-player games", Newell replied with a simple "yes". But then things really got interesting.
A user later asked "Any chance of a new IP that takes place in the Half-Life/Portal universe?" and Newell replied with a simple but assured "Yep."
Details on what this game (or possibly games) entail are slim, but Newell offered a few teases in terms of the studio's current thinking.
When asked "Is Valve interested in making a full game experience for the Vive?" Newell replied, "Yes. We think VR is pretty important as a tool for interesting games." This suggests that whatever this mystery game is, it will be in VR. Unless it's actually two different games, with one in VR and another set in the Half-Life/Portal universe.
Of course, this may not be as exciting as one thinks, since technically Valve's HTC Vive demo The Lab is set in the Portal universe, even if it's not the next major installment of a beloved series.
It's worth noting that Newell was also replying to a question about a new IP, so it's doubtful he's referring to Half-Life 3. Sorry.
Newell further noted that Source 2 is still the studio's universal engine for software development. "We are continuing to use Source 2 as our primary game development environment. Aside from moving Dota 2 to the engine recently, we are are using it as the foundation of some unannounced products," Newell said.
"We would like to have everyone working on games here at Valve to eventually be using the same engine."
"We also intend to continue to make the Source 2 engine work available to the broad developer community as we go, and to make it available free of charge," he added.
Elsewhere in the AMA, Newell confirmed that the the two movies Valve announced that it would be making with JJ Abrams are still in development. "They're coming," Newell teased.