This could be fun, provided that you don't model the 'disabilities' on real-world disabilities - if you make them abstract, related only to game mechanics. I mean, the bottom line - best case scenario - is that the use of real-world disabilities will be the glaring feature of the game, making the actual game itself an attached novelty.

Politically correct BS aside, games don't feature disabled players because no one wants to be disabled. People want to be bad-ass space marines or deductive powerhouses with top hats and boy sidekicks. You might use a disabled player character to make a statement about disabilities in a game, which is entirely different than making a fun FPS.

A kind of head-over-heels game with multiple players in 3d would be cool.

For a blind person you could just enhance the audio cues in the game and use minimalistic visual cues to indicate bumping into something, etc.

You could give them a stick, controlled by the mouse (always visible since a blind person would be able to perceive the position of the stick relative to themselves) and when you bump into something use a visual clue to give the player the message (obviously, vibration would be helpful here, but most PC gamers don't have peripherals that vibrate).

Additionally, enhance their sense of sound by letting them more easily hear players and the environment around them.

Moving on, I nominate "woman" to be a character class in the game.

I was referring to the 'old people' glasses that have a strong tint since light without them actually is blinding and hurts them. Perhaps in the game they could be jostled off and cause damage around light sources like the sun. Unless a button is held to 'close their eyes.'

Maybe there can be a simple 'elder' combining 'bad-back' and 'eye sensitivity.' Combining multiple would be fun, like if the ADHD character coincidentally had glasses or something. Don't use that example.

Maybe combining ailments until a certain level of handicap is reached. I also still don't know how the classes would be chosen without any specialties for any.

You would do good to have 'special skills', passive or activated for the characters, IE the blind person could passively effect the range of a radar with heightened hearing whereas a paranoid/SAD character could install a limited amount of locks on doors to keep enemies out of (or in) an area. A wheelchair-bound character (alone or with another character's help) could manually initiate a full-speed ram to locked doors/enemies for varying strengths depending on the character pushing the wheelchair, if any.

Developing key skills and dependence on other classes is key. In Resistance 2 there was a standard soldier class which had a limited amount of impenetrable shielding power to allies, a medic that could heal itself and others and revive faster, and a spec-ops/bandolier to other classes, and could replenish the soldiers shield and the medics 'healing orbs.' Normally there was a 20-second respawn time and the mission failed if everyone died. he medic revived in like 2 seconds and others in around 10 from the full-wait of respawn time. All the classes needed each other.

Noobs do this in Resistance 2, but surprisingly most of the time there is tons of teamwork, in fact when each of the classes are in formation, literally nothing can stop you. (Unlimited healing, shielding, and firepower in best formation)