You're entering complex territory! I am about to return to freelance, but for the past couple of years, I have been working on apps that are used by people with a number of diagnoses or acquired conditions. A couple of very broad statements I can make are that the more customisable an app or product can be, then the more fitting it will be. There are obviously plus and minus points to allowing for this level of customisation, so complexity has to be a factor that you weigh in. Autistic people may stim, which means that they will become fascinated and stuck on one thing, for apps a button that you tap which causes something to happen may cause stimming - for instance, if you tap on the 'home' icon on the Twitter iOS app you will see that it pulsates slightly, this subtle interaction may be fascinating and cause stimming, being able to turn off that interaction may be a consideration. A final comment is that a very subtle change may seriously upset some people, in one app I worked on a button moved slightly to the left (a matter of about 12px), this caused some discomfort to one user who was so used to tapping in one area of the screen that she wrote to ask if it could be moved back - although this particular person was just mildly upset, this seemingly subtle change could cause a serious meltdown with others.