Retro

Mod artist/level designers, check out the art/level design of Amiga classic Alien Breed series (I recommend Alien Breed Tower Assault) before doing your stuff. Believe it or not, there was a time that 3D cards/games didn't exist yet. The games that were 2D are well done (energy was focused on art/level design mostly). There are classics among them to take example from. Thx.

How about the original Doom?

That's the most obvious people will take reference from. No doubt we'll be seeing a lot of old doom maps redone.