Portal 2 expanded upon the original in many ways, including its addition of co-op to the obligatory single-player campaign. According to Valve itself, Portal 2 might very well be the last time we see the company offer an "isolated single-player" feature in one of its games.
As a part of the feature The Final Hours of Portal 2 (which was recently released as an iPad app), Geoff Keighley remarks, "Portal 2 will probably be Valve's last game with an isolated single-player experience." That's a bold, somewhat surprising statement to be hear about the company best known for the Half-Life series. And it's not one that was based on speculation.
Keighley clarified where that information comes from over the weekend, writing, "This is coming from Valve, although I don't have a lot more to share other than what is in the story. I agree it's a provocative statement, especially given the amazing single player storyline in Portal 2. Keep in mind Valve said 'probably,' so I get the feeling that this could change."
Beyond being provocative, it's also somewhat ambiguous -- does this mean no Valve game will ever again have a single-player campaign? Is Half-Life 2: Episode 3 safe, and if so, could it be single-player if it also had a co-op option or shipped with an updated version of Half-Life 2: Deathmatch? It's unclear exactly how to take the statement, but there thankfully is the saving grace that Valve qualified it by saying "probably."
Valve is currently at work on Dota 2 and possibly something involving this concept art. Given the company's nature, don't expect it to open up on any further on future plans until it's ready to announce its next project.