Also one could argue that all 3D graphics are illusory since the monitor is basically just a m,n-matrix of pixels. The renderer performs a transform on the (x, y, z) co-ordinate of each point to generate the (x, y) index into the matrix ("index into the matrix" sounds way cooler than it is... now I'm imagining a game where you have to interact with The Matrix from the outside using debugging tools or something).
Doesn't the "first person" part require 3D graphics?
Not necessarily, it could be something as simple as using a mouse to move a crosshair over a flat plane where enemies jump up from behind cover. I remember a few flash games like that. Obviously that means characters can't move towards or away from the player, but they can be positioned and scaled so as to give some illusion of depth, which was the case in the flash games I played.
We can brainstorm gameplay ideas in a separate thread if you like. It could be a new community project like Chess++ (whatever happened to that, anyway? I forked the repo but never got around to contributing because I was busy with my own projects).
A model is created in any 3d modeling program. Along with the actual model, one creates a bounding box around the model ( a cube by default ). To export into the game, 20 pictures of the model are created, 1 for each zoom/rotation combination. The pictures are mapped onto the bounding box. The bounding box is rendered in-game, so there is an "infront" and "behind". However, it's a trick because you're also looking at a sprite ( the picture that was taken ). Not really 2D, not really 3D.
I might have argued that this was the first 2.5D game, but I don't think I will :)
You can move in the back and front of Little-Big planet, so that technically eliminates that from the list based on that definition.
In IGN's review, Chris Roper also reported issues with the control system stating that the game's heavily physics-based gameplay "left a few corners on what should have been a razor-sharp control scheme". He goes on to say that the character acceleration and deceleration "isn't as quick as it could be" but that his biggest complaint is the way the game handles player movement between foreground, middleground and background on the 2.5D plane. He says that "there are instances where it doesn't do what you want it to do, and these points stick out like a sore thumb."
In any event... regarding OP's question... how could you have an FPS ("first person shooter") without having 3D graphics? Doesn't the "first person" part require 3D graphics?
You use 2-D Sprites and mess with the scale to make them appear bigger or smaller. You set a maximum scale to represent them "running into you" and you use the X and Y of their current local bounding box to determine if your shot "hit" or not.
I started doing a proof of concept with this idea in SFML awhile ago and I even had a haphazard melee system for it. But I couldn't get the angle on the walls to appear just right, probably because I was using MS Paint.