Handicapped sims. Sims can be made blind or def. They can also be made with missing limbs. Some will even use wheelchairs. Some sims may become handicapped as a result of injury or disease. Or they could occur during age up.
This supplement would also include build items such as ramps, hand bars, small elevators, braille resources, home medical beds, specially constructed game controls, and many of the other inventions that help handicapped people in daily life. Character creation would include wheel chairs, prosthetic limbs, stumps, cane accessories, hearing aids, and more. Household services could nurses and rehab teachers. Necessary skills could be learned on rehab equipment that can be found in many gyms. Plays with the Cats and Dogs expansion can also add service animals to their household.
On a sidenote. People with mental illnesses may also be represented by this expansion. But I understand if Maxis doesn't want to open that particular can of worms.