Why don't I have a custom title by now!?
A little bump. Come on people and share your thoughts, it is so beautiful to dream... :)
Should Doom 4 base itself on Doom 2, craft a new story, or a combination?
While I'd be happy if the story was based on the Doom2's, I think id would better not recycle it exactly, keeping references to Doom 2 to a minimum. In one sentence - just don't exaggerate it like Duke Nukem Forever did.
How much emphasis should there be on demonic aesthetics?
I definitely think they have their place in Doom, since - it is Doom. Satanistic imagery, if used in gripping moments to complement overall unknown extradimensional forces, would amazingly add to the atmosphere. In original Doom, if you think about that, demonic stuff also isn't overused, but the developers used it to create an atmosphere - at least we must admit they tried.
How much emphasis should there be on story?
Rather low, since Doom is (should be) oriented primarily on another things. No need for interesting twists, side stories, elaborated psychology of characters etc. If they decided to go this way, I'd take it as a bonus to the shooting fun - if the story will disturb me from that, then I'd lament.
How much emphasis should there be on modern features such as alternate fire, vehicles, destructible environments, NPC interaction, inventory items, etc.?
I'd be more satisfied to not see these in Doom 4 in large scale - driveable vehicles just don't fit, however if they'd perform them well and in an interesting way, I wouldn't complain. Destructible environment in minimal measure. Also, they might come up with they own new features, that are derived (not copied, but even that is standable) from old Doom game mechanics - like secrets, secret levels, Invulspheres ;)
How futuristic or archaic should the weapons be?
Speaking about one specific example - I'd like to see a wooden shotgun similar to the classic one, for the satisfaction from using it to kill classic-like imps (sorry for overtaking a little). In general, I'd prefer a contrast of (mostly low-power) archaic vs. (mostly high-power) futuristic weapons, like it was in classic games too.
New monsters, old monsters, or a mix; and should they follow the art direction of the classic games?
Again, I would be really grateful for naked creatures with (dark-)brown skin, horns growing from different body parts, not necesarrily red eyes, human-like legs and arms with which they're throwing fireballs at you, and with just the health to be killed by one shotgun shot from quite a distance. Yeah, I'd definitely prefer returning to the classic style - while modifications making them look horrific would be cool too. But I realise how difficult it'd be to perform such modifications well.
How should multiplayer be handled?
Online coop support and additional built-in deathmatch maps that don't lag. These might try to emulate "classic level design style" by their abstraction, height variation and mechanics - but this is just wishful thinking.
How linear, complex and realistic should the maps be?
Nonlinear, with low attention to graphics and realism and greater attention to fun gameplay, sized to take you 15-60 min each, depending on how thoroughly you want to explore it.
What are your expectations concerning the engine's capabilities?
All graphical features, physics, light effects and impressive environmental changes aside, I don't rely on them. The engine must be able to handle many awakened enemies on the scene and not only on the scene but also moving around the map elsewhere.
What should be the approach to sound design?
Monster sounds not necesarrily based on the classic sounds, maybe it'd be even better to avoid it and make it rather more horrific. Using realistic and/or moody (in any case, appropriate) ambient sounds *might* (to some extent) compensate lacks in graphics, which (lacks in graphics) I personally wouldn't mind either way.
Should the player receive NPC assistance during the campaign?
Yes, NPCs should definitely make appearance and interact with the player, but only sometimes. And the word "assistance" is important - the player should be forced to cooperate with them, not that they do all the job for you (a la Sentry bot - well, *most* of the job) or do needless job (a la scientist with a lantern). In addition, there may of course be people who give you something valuable, or people just desperately trying to survive and WHOSE YOU CAN SAVE (in D3 and RoE they kind of overincluded people dying in front of your eyes while you cannot do anything, and I didn't like that).
How much emphasis should Hell have?
Well-worked aesthetically amazing environment looking abstract and like something that might have come from another dimension, and is continuously growing bigger and bigger and taking control over everything on Earth. The presence of Hell must be something which will make Doom 4 distinguishable from other FPSs, in the sense of design and atmosphere and ideally also gameplay.
I also expressed my expectations about Doom 4 (or ideal Doom remake, in fact it means nearly the same for me) here.