One of the biggest problem with current and indeed future disability related benefit is that it tries to measure disability, as oppose to impairment, and then try to put absolute lines between who is disabled and who is not disabled, where a fixed uncalculated amount of money is provided to those who cross the line regardless of their individual needs.
Now measuring impairment is hard enough and trying to compare impairments is near impossible as what basis do you measure from like resources needed, % of body/mind affected or emotional experience. For the later, the level of impairment may have very little to do with how someone is coping.
Measuring disability means the social effects of impairment of a specific range of activities depending on the benefit. By working on what people can and can not do, its using abstract situations that end up making no sense. I can achieve any outcome I wish in my own method and with the right support.
I would therefore scrap all disability benefits and replace them with a single positive assessment system that supports people with individual payments to assist them to meet their outcomes. So it is not about what you can or can not do but what do you need to achieve their outcomes.