At 14-16 years old I don't expect @Delta5630 to be able to bring a product to market.
Putting that aside, I would say the very first thing is to look for any communities of disabled players then ask them what they need.
Or observe them and surprise them with something they didn't know they wanted/needed.
Mental and Physical disabilities are wide-ranging and complex. But we can attempted to break it down to a few categories, leaving out ones that totally prevent one from playing like blindness.
Arms
Legs
Please suggest more.
Can they use one or both arms? Same for legs.
One arm = Steadycam solution
No arm control = that mouth controller (anyone know the name?) connected to a motorised gimbal. Maybe get a rail attachment to tripod mount for a pistol. Just ideas.
One leg = multicam tactical prosthetic leg
No leg control = tank built on a gokart with a wooden body (much cost less than half of ours)
Even if you can't make it, we appreciate out-of-the-box thinking, so let us know what you come up with.