There have been numerous Doom texture projects, each more faithful than the last. The problem is that one you apply those textures to the default levels, they look very weird because they are much higher quality than the environment, therefore they look out of place and break the coherence of the level. It's not as simple as making better textures. When you upgrade something in a game, I think everything has to be upgraded: from textures, models, animations and level geometry itself.
I do and I dont agree with this, in my opinion high res replacement textures 'do' look very good in Doom, but many texture packs are made by various contributors all with their own art style. So even though it does look better, it sort of looks out of place. In Doom Ascension, every texture goes through NiuHaka and its tweaked to hell so it fits DA art style, this keeps consistency. However, this causes vanilla monster to be out of place, especially when up close to a high res wall or dead on a high res ground. So the monsters need upgrading! Personally I think the only way to do this is to go 3D.
As for the 3D models, many port support 3D models and yea packs have been made, but they are old now and their poly limit was limited to the early engine versions/hardware at that time. However, a higher poly monster model made properly (still with sensible poly limits) and a high res texture using the 'same' style the walls etc will look much more at home. Then of cause there is the animation! This is what can make or break a good looking monster. Its not easy to animate a humanoid, your either naturally talented or have had proper training.
With these things in place I think Doom can feel and play like the original, but with a more modern look.
Finally, as for environmental models to use in levels to make the 3D monsters suit their environment, yea definitely, think Doom really needs this to make more sense of the levels purpose and add a more modern look, hopefully without effecting game play too much. (This is actually what I am working on right now and its looking nice)
All this is exactly what Doom Ascension is doing :)
However, remember not everybody will like using 3D models, many just like the pure vanilla game play.....its all a matter of preference.
There is however, a major difference between 2D and 3D at that level of detail: labor-intensivity. Making a good 3D model is already a full-time job, making some hundreds of hand-painted sprites WITHOUT resorting to using models would take a lifetime. If you end up using models anyway, why not go full 3D and benefit from infinite rotations, flexible posturing, smoother animation, lower memory footprint and hardware acceleration? Get over it, pure 2D for such high-res gaming applications is dead.
Totally agree with this, think the end result will never be as good looking as a decent 3D model. Also im guessing if something simple like a walking animation needs tweaking, then thats a massive change! With a 3D model its a simple tweak, and re-import....done! Hence much easier to get things looking right. DA animations are often being tweaked all the time. Its often the minor things that make a difference!