The one thing i would like before anyone becomes a parent is a big investment of counselling services. Most people don't recognise why they shouldn't become parents, yet.
Practical tests are a waste of time, as you cannot test whether you would emotionally damage a thing that relies on you for their total emotional needs and development and the test would have to involve wider family.
A contract would have to be signed that you would never have another partner or regular visitor to your home, unless they passed an upto date test.
Speaking as a CP SW and someone who removes children, monitors parenting and carries out assessments and recomends interventions, you could not come up with a "blanket" set of testing.
The right to not be medicated against a natural function is a Human Right, as well as an ethical right.
Thankfully because of the Human Rights Act, it isn't something that we should fear.
If i set my test, i know from past threads that many on MN, wouldn't have passed them.