Lovely work!
If i were to throw my two cents (though most stuff was already said by caligari, who was absolutely on point) there is a huge problem you'll face, which is incredibly ironic in and of itself: You want to make this shader so animation and rotations are easier to do, but the more animations and rotations you do, the more the brain realizes this is, in fact, a 3D object.
The idea is that the brain isn't dumb. At all. In fact it is incredibly fast to detect depth in things.
For example, in the unholstering animation you posted, that alone breaks the ilusion: everything moves like it should have, everything gets smaller or bigger depending on the angle: i can see that it is 3D. Which kind of sucks! There goes the irony.
Showing perfect depth and placement in character rotations does this as well.
Usually there is a method to go around this. Severely limiting animation frames is one- for example, Guilty Gear (the ones that are 3D but look 2D) limits it's poses when attacking, and depending on the position of the camera, move certain parts of the characters (like mouth, eyes, that kinda stuff that are mostly drawn depending on the angle) to fool the brain. It mostly works!
What i have seen people do, with 3d to 2d weapons, is to limit frames like this. Usually 1 frame idle, maybe 1 when lowering, and to make it stiff when shooting, rather than having many frames. You are allowed to do many frames with the reload, but be careful, as too many angle changes can lead to the brain realizing.
Lately i saw this tweet from a dude making something akin to this for his guns, and i think it looked rather neat.
https://twitter.com/JasozzGames/status/ ... 7295331329
He has a different artstyle than Doom, though, of course!
As for the art itself, make it
-not have banding (this automatically kills the effect), use hand drawn details in some places to better fool the person looking
-avoid poses which look too 3D (way too subjective, but w/e, arms or legs way too open, things like that, as long as the legs don't go all over the place like the arachnotron you are good to go)
-make poses look flatter (by this, i mean the field of view, i guess, if one leg looks way smaller because it's further back, it won't look like fake 3D, it will look like a rendered frame from a model)
-be careful with shining and lighting (sometimes, you might have your renderer options all correct, but the absolutely correct lighting, which is the best way for our eyes to see depth in 2D images in real life, will be an obvious tell that it is a model, as drawings usually have imperfections that, while you might not be able to tell, your brain actually does tell. I have no idea how to properly fix this. Baked in lighting? That could be problematic if not fixed post rendering. hmmm)
-ugly messes. By this i mean that, sometimes, when downscaling, a model suddenly looks all ugly and weird. Lines become blobs, faces become whatever, parts of the body become flat, contrasts become blurry. This is bad. Pixel artists usually spend hours looking at the same set of images, and work pushing pixels one at a time, they see these things. After all, pixelart is a way to convey more detail with less. Therefore, there are tricks to do so- playing with the pixels in order to suggest a weapon and its small details, a face with its nose and eyes and mouth. It might only be a few pixels, but these small details are what are important, and downscaling can either make them dissapear or become messes.
I guess a perfect sprite shader would check on those small details and make them pop? No idea.
I see these kind of projects as an awesome jumping point, however, as you can give these frames to an artist and he can finish the work by cleaning them. Like Ion Fury, i guess!
This is all if you want to replicate doom's style, however.
After all, one thing i always say is that it doesn't matter how it looks, the thing that matters is that it looks *consistent*. If everything looks the same way, it's a style. If it all looks different, it's a mess, hahahaha