While Bethesda and developer id Software are keeping quiet about the future of the "Doom" franchise, id's Studio Director Tim Willits revealed that what would have been know as "Doom 4" ended development because it "did not have the spirit, it did not have the soul, it didn’t have a personality."
Speaking with IGN's Mitch Dyer during QuakeCon, Willits called the defunct project "schizophrenic," lacking the personality of the long-running shooter franchise. The problems with that project were, Willits says, hard to articulate, but owed to a collection of things that didn't let it match the quality of previous entries in the series.
While there's been no formal announcement of the next "Doom," Willits' comments offer hints that the developer remains in active development of a followup to 2004's "Doom 3." Willits says that for the new project (which he won't call "Doom 4"), the developers have cut any "distractions" from the team and that "all hands are on deck" for the sole major project currently in development at id.
Last year, Bethesda and id repackaged "Doom 3" in the "BFG Edition" which added more levels, HD visuals, and 3D support.
Interesting FYI: December will represent the 20th anniversary of the release of the first "Doom" on PCs.