You could go down the sensory route. Take a toy and add extra sensory effects - so for instance, you could use a different material for each different colour (
http://www.sensorytoywarehouse.com/ has some good inspiration) These toys would be good for children with autism spectrum disorders. Another idea would be to modify something, say a toy car so that it is indestructible (made of strong rubber, no small parts), which would be good for a child with Pica. Hope these give you a bit of help. One way to help might be to pick a disability and look for problems with the toys that are available and then fix the problems.