You know, for all the indie games that have been made that try to emulate the look of old games, I don't think I've seen one that tries to emulate the look of old-school FPS's that are 3D but use 2d sprites as enemies, ie Doom, Heretic, Hexen, Duke Nukem 3D, Blood, Powerslave, Witchaven etc).

Why is this?

I really have no clue man.

There's Delver:
http://delvergame.com/
and there was another good quality FPS with 2D sprites, but I can't remember the name right now.
But many others skew more towards the technical level and simple gameplay of Wolfenstein rather than Duke 3D:
http://www.indiedb.com/games/starshock
http://www.indiedb.com/games/scary-gardener-tales-3d
http://bunkerpunks.com/
http://store.steampowered.com/app/295770/
http://www.vlambeer.com/index.php?tag=gungodz
http://www.locomalito.com/8bit_killer.php

Delver is great! Enjoyable in every category, in my opinion.

If you want a FPS then check out In The Kingdom. http://amon26.itch.io/in-the-kingdom
It has the art style down perfectly, it's fun too although the level design leaves a lot to be desired.

I imagine its harder to write an game using both techs. Imagine lighting and clipping, the 2d sprite would need some sort of normal map or hull so that lighting and clipping would work. Also, most animation is done using the 3d model and then if needs rendered to 2d. Saves work just saving the model and using that.

Can't type links at the moment, but I'm currently playing a standalone WOLF3D total conversion called Orb of Dilaria. It's been cool so far, fantasy themed with branching level paths.

http://www.locomalito.com/8bit_killer.php

*edit*: Oh, now I see that someone has mentioned it before. Sorry!

I remember that one. Is it possible to play it via ECWolf? Last I remember it was only dosbox.

Because it always looked like ass. I don't think there's any way of making that look good. Plus, it's a ton of work. you need to get the item from multiple angles and it never looks good.

OTOH, those old pixel games did look pretty good in many cases. By the time the NES hit the market, the games could look pretty good, even though they were limited to 8-bit color and had low resolution. That's something that can be scaled pretty easily into something that still looks good whether you stick to 8-bit color or move to 16-bits.

I tried playing it with ECWolf and it didn't seem to work, though I'm also not great at setting up ECWolf, so don't take my word for it. I just downloaded the packed DosBox player. Not great, but yeah I dunno what other choice there is.

http://store.steampowered.com/app/295770/

No, you can't play it with ECWolf. I did Orb of Dilaaria before ECWolf existed and haven't looked into what would make it compatible. There are a lot of changes in Orb, though I guess I could diff it against the original code to find out everything I did.

I've wondered about doing 2D sprites in 2.5D/3D games, too. I thought that creating 3D models, taking shots to produce the needed 2D images, and then cleaning up the 2D images (something similar to what they did for Doom, I believe) would be a nice way to do them. At least it seems that would be much easier than making 50+ images from scratch per actor.

I recall lots of 2.5D FPP games used this technique for characters, you can see with games like Duke 3D, Blood, Shadow Warrior, Realms of the Haunting, and Normality. As you say, it's much easier and gives more consistent and uniform results than hand-drawing dozens of frames per character.

I'm not so sure about that. With sprites you have to do each one from scratch and multiple versions for every direction you might want to have. With models, you get to recycle a lot if you need to, plus it's one model per whatever no matter how many directions you like.

Also, you've got to hand draw each frame that you like. Which is a pretty substantial amount of work that has to be duplicated.

If you've got radically different enemies, then yes it is a bit more work, but if you're dealing with many different humanoids, then you should be able to recycle a large amount of the work.