I've been thinking about it a lot lately and it's clear to me that id should take the following approach:
1. Implement, in a modern fashion, the elements which made the previous "Doom" games great.
2. Learn from past mistakes.
3. Understand the contributions made by the community and implement at least some of those features.
#1 is pretty obvious. Fast player speed, dozens of simultaneous enemies, constant action with very little plot development, straightforward gameplay, linear but exploratory level design, and a unique mix of themes: Militant, industrial, archaic and demonic.
#2 is much more subjective. "Doom 3" is a good game but id dropped the ball more than once, resulting in slow player speed, flimsy weapons, repetitive maps, non-threatening enemies, a deemphasis on Hell and demonic abstraction, forgettable music and cramped spaces. Then there's Rage, which I have not played, so my only comment there is for id to work out the kinks with the texture buffering issues, and to improve the lighting, which in some ways was a downgrade from "Doom 3."
#3. The community has developed a deep love for the PS1 and N64 versions of "Doom." id should take note of the fact that the eerie lighting, dark ambient soundtrack and the transformational sequences ("Doom 64" only) made for a highly immersive experience. Additionally, mods such as "Brutal Doom" are a strong indication of the kind of brutality fans are expecting. The fact that vanilla "Doom" is still used prominently in mapping has made it clear that fans love the bizarre abstract reality imposed by the original engine.
I'm sorry, but I didn't find Brutal Doom interesting enough to warrant a full length game.
The thing that irks me the most is that on one hand, I like the oldschool Doom gameplay. But on the other hand, if they were to implement these things into a modern game I think alot of these elements would come off as something "just to please the fans", or just out of place.
I think alot of these oldschool gameplay elements that we know and love in the classic Doom games are something that id should respect, but not copy directly. Like having a absurdly high walking speed for instance; there are different ways to create fast paced gameplay, and I've seen Doom 3 mods that demonstrate this.