He did reveal that the hardest part of tackling a new Doom has been for id "deciding exactly what the essence of Doom is," as such an endeavour is "heck of a lot harder than you might think."
Carmack also touched on the pitfalls of development and that one thing he'd loved to have changed was to "do more things more often," referring to long gaps between releases.
"The worst aspect of the continuing pace of game development that we fell into was the longer and longer times between releases. If I could go back in time and change one thing along the trajectory of id Software, it would be, do more things more often. And that was id’s mantra for so long: “It’ll be done when it’s done,” John Carmack told .
"And I recant from that. I no longer think that is the appropriate way to build games. I mean, time matters, and as years go by—if it’s done when it’s done and you’re talking a month or two, fine. But if it’s a year or two, you need to be making a different game.”
Doom 4 then certainly wouldn't fit into Carmack's revised id Software: "It’s been hard—one of the things that was a little bit surprising that you might not think so from the outside, but deciding exactly what the essence of Doom is, with this 20-year history, is a heck of a lot harder than you might think. You get multiple Doom fans that have different views of what the core essence of it is, and there’s been a design challenge through all of it,” he admitted.
Doom 3 took a long time to reach fans and now Doom 4 threatens the same, which of course holds up id from pursuing other franchises. Hopefully Doom 4 won't suffer the fate of Duke Nukem Forever which failed to meet expectations after a painfully slow development. Check out thebetween John Carmack and Wired.