To design for non-stigmatization is really hard, mostly because it’s the society that creates the stigmatization, not the stigmatized individual. And as we all know it isn’t the easiest thing to change the society.
One impairment that isn’t visible but still leads to stigmatization is deafness. One example where design could help is in shopping malls, airports and other places where information is communicated with the customers through speakers. In these places, screens could be added that are visible throughout the premises that show the messages spoken in the speakers. This would also be good to reach people wearing headphones, listening to music or other stuff.
Speech recognition is another tool that could be used to aid those with hearing impairment, given that that technology is accurate enough. This would at least simplify the communication in one direction in public places like the grocery store or the bus. I imagine small screens in front of, for example, the bus driver, printing what he or she is saying. How the deaf person should communicate back in a non-stigmatizing way is a problem to solve though.
Text-based communication is an area that is the same for people with hearing impairments and people with fully functional hearing, so using technologies and devices that supports this type of communication definitely broadens the “normality” area.
Non-excluding Design 
I like the discussion about the society creating the stigmatization. To understand that it takes a long time to change the mindset of people in the society. One conclusion of this could possibly be to increase the visibility of that which is not “normal” and that way increase the awareness among people.