The thing I didn't like about Doom 3 was that it still felt like I was playing the first doom mechanically, and atmospherically a generic shooter. It lacked all the control and flexibility of the more advanced mechanics of shooters that were coming out at the same time and didn't bring anything else to the table.
If they were to make a Doom 4 it would have to feel like I am moving a character around shooting at things, not hovering some gun above the ground arbitrarily, flailing about in stiff arcs trying to aim at things. Those controls were great for the first 2 because that sort of experience was so new it wasn't hard to make you feel like you were walking down a hall shooting demons, but now that we have gotten better experiences with the feel of movement that old crap just doesn't cut it and it is fake nostalgia, it captures the mechanics but not the experience.
I don't think they need to do much with "horror" elements, it was a game of murdering endless masses of abominations, and I'm ok with sticking with that dynamic. In fact, I would worry that if they worked too hard on "horror" elements it'll be largely indistinguishable from other "horror" space shooters like dead space. Also, I'm tired of shooters in space ships/stations, can we go the route of the original Doom and make it on a planet so it isn't just all endless steel hallways? I don't buy that dynamic half the time when it is on a space ship, who's gonna design like that it'd have terrible issues with maintaining temps because bare steel is the opposite of insulation, it'd probably drive someone crazy with the lack of round edges and soft surfaces, I don't know, if it isn't just a beautified doom1/2 in a generic setting then I'm all for it, otherwise RIP Doom.