It was recently announced that id Software's next big game would be a whole new license: not DOOM, not Quake, not Wolfenstein, and no, not even Commander Keen. During Carmack's talk, he also made it clear that the company doesn't want to spend another four years on their next title. Carmack estimates that he'll have the game engine renderer done in less than a year. He also estimated that the art team will be able to start creating media for the new engine within the next month. Things are moving fast!
The next game for id Software will be a departure from anything they've done before, but we did learn a few things. For one thing, it'll focus on single-player, much like DOOM 3. Carmack said that this tended to be a good model -- in a single-player game, framerate isn't quite as important, so a state-of-the-art engine that really pushes systems to the limit isn't as much of a drawback. Then, partner companies can come out with multiplayer-focused games using that same engine shortly thereafter -- much like how Raven Software is doing Quake 4 using the DOOM 3 engine, focusing on multiplayer.
id's next title will be a first-person game, and although it will focus on single-player it'll have a basic multiplayer such as DOOM 3. What will it look like? What will it be about? Sadly, no details were forthcoming. We expect an announcement in the coming months.