Currently all forms of mental illness are diagnosed based on symptoms. There is no biological test to confirm any DSM-V diagnosis. That will change. There have been large genetic studies done that show that what we think of as different disorders actually share genetic material. For example bipolar disorder and schizophrenia. So in the future, I predict there won't be new disorders but the way we think of existing ones will change. I think it highly likely that the biochemistry of psychosis is the same regardless of what disorder it results from. There is compelling evidence that psychosis happens when dopamine receptors get overstimulated. The future is not in refining our classification systems. It's making the leap from mental illness to neuroscience.

 

Also, addiction is known to be a brain disorder and probably the disorder that causes the most suffering out of all of them. Unfortunately very few good treatments exist. The best treatment is abstinence but getting there is problematic.