So, how do you figure out what's realistic for your child? It's best done in three simple steps.
1) Trust Parental Intuition
You know your child better than any professional can. You've seen your child grow up, and you have a good idea of his or her strengths and weaknesses. To improve the power of that intuition, get educated about your child's diagnosis. Just be careful, because there's a lot of misinformation on the internet. Find sources you can trust and avoid anything that takes a strong "black or white" tone, because that's (usually) wrong. Then, if a professional tells you something that doesn't seem to fit your child, consider getting a second opinion.
2) Take It Step By Step
It's not necessary or even possible to plan your child's entire future. Even before the diagnosis, you weren't able to do that. With any child, it's best to aim for progress, and give up worrying about what the end point will be. Set small, concrete goals and look for ongoing improvement.
3) Pay Attention to Your Child
If your goals are too intense, or your expectations too high, your child will probably let you know. But the signs might be subtle, so you need to pay attention. If your child seems overly nervous, depressed, withdrawn or volatile, consider what might be going on. Are your goals and expectations putting your child under too much pressure? Consider easing up for a while and see if things improve.
As an example, many parents are told that their child with an autism spectrum disorder will not have "good relationships" as an adult. It's okay to question that statement based on what you know about your child specifically. Start with the knowledge that many people on the autism spectrum are happily married, or parents, or in other satisfying relationships. Factor in what you've seen for your child. Has your child been able to get very close to a teacher, friend or family member? That's evidence that the professional opinion may be too limiting.
Then go to the second step and aim to improve your child interpersonal skills. Don't worry about the end goal of what your child will achieve as an adult. Just try to improve current social skills, looking for small progress in things like relating, coping with emotions and being thoughtful.
For step three, see how your child is reacting. Almost everyone loves to achieve and succeed, as long as the pressure isn't too intense. Enjoy the successes and the progress along the way, and don't worry too much about the end point.
The final point to remember is that for many kids, progress comes slowly, but things do keep improving. In Engaging Autism, by Stanley Greenspan, M.D. (De Capo Lifelong Books, 2006), there is an excellent chapter on working with older children, teens and adults, subtitled "A Lifetime of Learning". Greenspan, best known for his development of the DIR/Floortime approach, argues that it's important to "overcome the myth that children reach a developmental plateau beyond which improvement can only be minimal. In fact, during the teenage and adult years, the brain and nervous system are still developing." (p. 212) Give your child lots of time to fail, to try again, and to succeed.